UNIVERSITY  OF  CALIFORNIA. 


FROM   THE    LIBRARY   OF 

DR.  JOSEPH   LECONTE. 
GIFT  OF  MRS.  LECONTE. 

No. 


THE 


ELEMENTS  OF  GEOLOGY; 


ADAPTED   TO  THE  USE  O* 


SCHOOLS  AND  COLLEGES. 


BY 

JUSTIN   R.    LOOMIS, 

PROFESSOR   OF   CI1EMISTRT  AND   GEOLOGY   IX    WATEKVILLE   COLLEGE. 


"WITH 


BOSTON: 
GOULD  AND  LINCOLN, 

59  WASniXGTOX    STREET. 


Entered  according  to  Act  of  Congress,  in  the  year  1852, 

Br   GOULD    &    LINCOLN, 
In  the  Clerk's  Office  of  the  District  Court  of  ihe  District  of  Massachusetts. 


Stereotyped    by 

HOBART   &   ROBB1N  S, 

BOSTON. 

PRESS  OF  G.   C.  RAND,   CORMIILL,   BOSTON. 


PREFACE 


IN  preparing  the  following  work,  it  was  intended  to  pre- 
sent a  systematic  and  somewhat  complete  statement  of  the 
principles  of  Geology,  within  such  limits  that  they  may  be 
thoroughly  studied  in  the  time  usually  allotted  to  this 
science. 

A  sufficient  number  of  leading  facts  has  been  introduced 
to  enable  the  learner  to  feel  that  every  important  principle 
is  a  conclusion  to  which  he  has  himself  arrived ;  and  yet,  for 
the  purpose  of  compression,  that  fulness  of  detail  has  been 
avoided  with  which  more  extended  works  abound.  In  fur- 
therance of  the  same  object,  authorities  are  seldom  cited. 

The  consideration  of  geological  changes  is  made  a  dis- 
tinct chapter,  subsequent  to  the  one  on  the  arrangement  of 
materials.  It  should,  however,  be  remembered  that  these 
processes  of  arranging  and  disturbing  are  not  thus  separ- 
ated in  time.  In  nature  the  two  processes  are  always  going 
on  together. 

It  seemed  important  to  exhibit  the  science  with  as  much 
unity  and  completeness  as  possible ;  and  hence,  discussions 


IV  PREFACE. 

upon  debatable  points  in  Theoretical  Geology,  so  interesting 
to  mature  geologists,  would  have  been  out  of  place  here ; 
and  yet  those  more  intricate  subjects  have  not  been  omit- 
ted. A  large  proportion  of  the  work  is  devoted  to  the 
explanation  of  geological  phenomena,  in  order  to  convey  an 
idea  of  the  modes  of  investigation  adopted,  and  the  kind  of 
evidence  relied  on.  Where  diversities  of  opinion  exist,  that 
view  has  been  selected  which  seemed  most  in  harmony  with 
the  facts  ;  and  the  connection  has  not  often  been  interrupted 
to  combat,  or  even  to  state,  the  antagonist  view. 

Technical  terms  have,  in  a  few  instances,  been  introduced, 
and  principles  referred  to,  which  are  subsequently  explained. 
The  index  will,  however,  enable  the  student  to  understand 
them,  without  a  separate  glossary. 

Some  may  prefer  to  commence  with  the  second  chapter, 
deferring  the  study  of  the  elementary  substances,  minerals 
and  rocks,  to  the  last.  Such  a  course  may  be  pursued  with- 
out special  inconvenience. 

Questions  have  been  added,  for  the  convenience  of  those 
teachers  who  may  prefer  to  conduct  their  recitations  by  this 
means.  But,  when  the  circumstances  of  the  case  admit  of 
it,  a  much  more  complete  knowledge  of  the  subject  will  be 
acquired  by  pupils  who  are  required  to  analyze  the  sections, 
and  proceed  with  the  recitation  themselves  ;  while  the  teach- 
er has  only  to  correct  misapprehension,  explain  what  may 
seem  obscure,  and  introduce  additional  illustrations. 


LIST  OF  ILLUSTRATIONS. 


1.  Columnar  Trap,  New  Holland.     (Dana.) 

2.  The  four  divisions  of  rocks,  and  their  relative  positions.     A,  Vol- 

canic Rocks.     JB,  Granite.      1,  2,  3,  4,  Granite  of  different  ages. 
Cy  Metamorphic  Rocks.     D,  Fossiliferous  Rocks.     (Lyell.) 

3.  Granite  veins  in  slate,  Cape  of  Good  Hope.     (Hall.) 

4.  Granite  veins  traversing  granite.     (Hitchcock.) 

5.  Extinct  volcanoes  of  Auvergne.     (Scrope.) 

6.  Lava  of  different  ages,  Auvergne.     (Lyell.) 

7.  Strata  folded  and  compressed  by  upheaval  of  granite. 

8.  Favosites  Gothlandica. 

9.  Catenipora  escharoides.     (Chain  coral.) 

10.  Caryocrinus  ornatus.     (Hall.) 

C  Leptsena  alternate.     Orthis  testudinaria. ) 

11.  {  }(Hall.) 
(  Delthyris  Xiagarensis.  ) 

12.  Section  of  a  chambered  shell,  showing   the  chambers  and    the 

siphuncle. 

13.  Orthoceras. 


6  LIST    OF    ILLUSTKATIOXS. 

14.  Curved  Cephalopoda,      a,  Ammonite  ;  b,  Crioceras  ;  c,  Scapliite ; 

d,  Ancyloceras  ;  e,  Hamite  ;  /,  Baculite  ;  g,  Turrilite.    (Jlgassiz 
and  Gould.) 

15.  Trilobite. 

16.  Cephalaspis  Lyellii.     (Jlgassiz.) 

17.  Pterichthys  oblongus.     {Jlgassiz.) 

18.  Fault  in  the  coal  formation,     a  a,  layers  of  coal,     b  b,  surface  and 

soil. 

19.  Stigmaria  ficoides  ;  Newcastle.     (Lindley  and  Hutton.) 

20.  Trunk  of  sigillaria.     ( Trimmer. ) 

21.  Bark  of  sigillaria.     (Natural  size.) 

22.  Sphenopteris  crenata.     (Lindley.) 

23.  Pachypteris  lanceolata.     (Brongn.) 

24.  Sigillaria  levigata.     (Brongn.) 

25.  Lepidodendron  Sternbergii,  Bohemia.     (Sternberg.) 

26.  Calamite. 

27.  Heterocercal  fish.     Homocercal  fish. 

28.  Impressions  of  Raindrops,  Wethersfield,  Conn.     (Hitchcock.) 

29.  b,  Bird  tracks   in  the   Conn.  River   Sandstone,      a,  Consecutive 

tracks  ;  c,  Track  of  Cheirotherium  (probably  a  reptile),  Penn. 
and  Germany. 

30.  Section  in  the  Isle  of  Portland."    (Buckland.) 

31.  Apiocrinites  rotundus,  Bradford,  Eng.     (Miller.) 

32.  Gryphea  incurva. 

33.  a,  Outline  of  Ichthyosaurus  ;  b,  Plesiosaurus. 

34.  Pterodactyle. 

35.  a,  Diploctenium  cordatum  ;  b,  Marsupites  ;  c,  Salenia  ;  d ,  Galer- 

ites  ;  e,  Micraster  cor-anguinum.     (Jlgassiz  $  Gould.) 

36.  b,  Belemnite.     a,  Restored  outline  of  the  animal  to  which  it  be- 

longed. 


LIST    OF    ILLUSTRATIONS.  7 

37.  Cerithium  intermedium. 

38.  Murex  alveolatus. 

39.  Conus  concinnus. 

40.  Nummulite. 

41.  Outline  of  paleotherium. 

42.  Outline  of  anoplotherium. 

43.  Skeleton  of  the  mastodon. 
41.  Univalve  with  entire  mouth. 

45.  Univalve  with  notched  mouth. 

46.  Unimuseular  bivalve. 

47.  Bimuscular  bivalve. 

48.  Parallel    planes  of  cleavage  intersecting  curved  strata.     ( Sedg- 

urick.) 

49.  a  b,  A  vein  of  segregation  ;  c  d,  A  dike. 

50.  faults  and  denuded  strata. 

51.  Vertical  conglomerate.     (Lyell.) 

52.  Inclined  strata  in  Dorsetshire,  England.     (Buckland.) 

53.  Dip  of  strata. 

54.  Axes  and  valleys  in  disturbed  strata. 

55.  Curved  strata  of  slate,  Berwickshire,  Eng.     (Lyell.) 

56.  Folded  strata. 

57.  Slope  of  mountains. 

58.  Europe  at  the  Silurian  epoch.     (Guyot.) 

59.  Europe  at  the  tertiary  epoch. 

60.  Area  of  elevation  and  depression  in  the  Pacific  and  Indian  Oceans. 

(Darwin.) 

61.  c  c,  Coral  wall.     (Trimmer.) 

62.  c  c,  Coral  wall  above  the  sea-level  ;  c'  c',  Second  coral  wall. 

63.  Coral  wall  after  partial  subsidence. 


8  LIST    OF    ILLUSTRATIONS. 

64.  Atoll.     The    coral  wall    only    appearing.      The    original    island 

entirely  submerged. 

65.  Eemains  of  the  temple  of  Jupiter  Serapis,  near  Naples. 

66.  Detached  hills  of  old  red  sandstone,  Rosshire,  Scotland.     (Lyell.) 

67.  Section  of  denuded  strata,  Mass.     (Hitchcock.) 
G8.  Grooved  and  striated  surface  of  rocks. 

69.  Artesian  wells. 

70.  Segregated  masses  in  rocks. 

71.  Columnar  form  taken  by  basalt  on  solidification. 

72.  Layers  of  limestone  now  forming,  San  Vignone,  Italy.     (Lyell.) 

73.  Erosion  of  rock  by  the  action  of  the  waves. 

74.  Marine  currents. 

75.  Sediment  deposited  in  horizontal  layers. 

76.  Section  of  greensand,  Bedfordshire,  Eng.     (Lyell.) 

77.  Glacier,  with  lateral  and  medial  moraines,  a  a,  Terminal  moraines. 

78.  Iceberg. 

79.  Volcanic  Eruption.     (Trimmer.) 

80.  Fractures  produced  by  upheaval. 

81.  Fossiliferous  rock  altered  by  contact  with  granite. 

82.  Consecutive  changes  by  which  horizontal  strata  become  vertical. 


TABLE   OF   CONTENTS. 


CHAPTER  I. 

OF  THE  MATERIAL  WHICH  COMPOSE  THE  CRUST  OF  THE 
EARTH. 

Pa-re 
SECTION  I.— ELEMENTARY  SUBSTANCES, 11 

SECTION  H.— SIMPLE  MINERALS, 13 

SECTION   HI.—  THE   MINERAL  MASSES   WHICH  FORM   THE   CRUST  OF 

THE  EARTH, 16 


CHAPTER  II. 

OF  THE  ARRANGEMENT  OF    THE   MATERIALS  WHICH  COMPOSE 
THE  CRUST  OF  THE  EARTH. 

SECTION  L  —THE  CLASSIFICATION  OF  ROCKS, 21 

SECTION  H.— THE  PLUTONIC  ROCKS, 23 

SECTION  in.— THE  VOLCANIC  ROCKS, 25 

SECTION   IV.  — THE    NON-FOSSILIFEROUS   STRATIFIED    (OR   METAMOR 

PHIC)  ROCKS, 30 


SECTION  V.— THE  FOSSILIFEROUS  ROCKS, 


10  TABLE   OF    CONTENTS. 

Page 
SECTION  VI.  — FOSSILS, 5? 

SECTION  VII.  —  THE  TIME  NECESSAEY  FOR  THE  FORMATION  OF   THE 

STRATIFIED  ROCKS, 63 


CHAPTER  III. 

OF  THE  CHANGES  TO  WHICH  THE  CRUST  OF  THE  EARTH  HAS 
BEEN  SUBJECTED. 

SECTION  I.— CHANGES  WHICH  HAVE  TAKEN  PLACE  AT  GREAT  DEPTHS 

BELOW  THE  SURFACE, 07 

SECTION  II.  —  CHANGES  IN  THE  MASS  OF  THE  STRATIFIED  ROCKS,  ...  68 

SECTION  III.  —  CHANGES  OF  ELEVATION  AND  SUBSIDENCE, 73 

SECTION  IV.  —  CHANGES  ON  THE  SURFACE  OF  THE  EARTH, 85 

SECTION  V.— CHANGES  OF  CLIMATE, 88 

SECTION  VI.  — ADVANTAGES  RESULTING  FROM  GEOLOGICAL  CHANGES,  .  91 


CHAPTER  IY. 

OF  THE  CAUSES  OF  GEOLOGICAL  PHENOMENA. 

SECTION  I.— ATMOSPHERIC  CAUSES, 95 

SECTION  II.  —  CHEMICAL  ACTION, 97 

SECTION  III.  — ORGANIC  CAUSES, 101 

SECTION  IV.  — AQUEOUS  CAUSES,. 103 

SECTION  V.  —  AQUEO-GLACIAL  ACTION, 120 

SECTION  VI.— IGNEOUS  CAUSES, 127 


CHAPTER    I. 

OF  THE    MATERIALS  WHICH  COMPOSE  THE    CRUST  OF  THE 
EARTH. 

SECTION   I. ELEMENTARY   SUBSTANCES. 

THERE  are  about  sixty  substances  known  to  the  chemist  which 
are  considered  as  elementary ;  but  most  of  them  are  rarely  met 
with,  and  only  in  minute  quantities.  A  few  of  them  are,  however, 
so  abundant,  in  the  composition  of  the  crust  of  the  earth,  as  to  ren- 
der some  attention  to  them  necessary. 

Oxygen  is  more  widely  diffused  than  any  other  substance.  It 
is  an  ingredient  of  water  and  of  the  atmosphere,  —  the  former  con- 
taining eighty-eight  per  cent.,  and  the  latter  twenty-one.  Nearly 
all  rocks  contain  oxygen  in  combination  with  the  metallic  and 
metalloid  bases,  and  the  average  proportion  of  oxygen  which  they 
contain  is  about  forty-five  per  cent. ;  so  that  it  will  not  differ 
much  from  the  truth  to  consider  the  oxygen  in  the  earth's  crust  as 
equal  in  weight  to  all  the  other  substances  which  enter  into  its 
composition. 

Hydrogen  occurs  in  nature  principally  in  combination  with 
oxygen,  forming  water.  It  is  also  an  ingredient  in  bitumen  and 
bituminous  coal. 

Nitrogen  is  confined  almost  entirely  to  the  atmosphere,  of  which 
it  forms  four-fifths.  It  enters  into  the  composition  of  some  varie- 
ties of  coal,  and  is  sparingly  diffused  in  most  fossiliferous  rocks. 

One  of  the  most  important  substances  in  nature  is  carbon.  It 
constitutes  the  principal  part  of  all  the  varieties  of  coal,  as  well  as 
of  graphite,  peat  and  bituminous  matter.  A  much  larger  amount 


12  ELEMENTARY    SUBSTANCES. 

of  carbon  exists  in  the  carbonic  acid  which  is  combined  with  the 
oxides  of  the  metalloids  and  metals.  The  most  abundant  ot  these 
compounds  is  limestone,  which  contains  about  twelve  per  cent,  of 
carbon. 

In  the  neighborhood  of  volcanoes  sulphur  is  found  pure  and  in 
a  crystalline  form.  It  is  a  constant  ingredient  in  volcanic  rocks, 
and  in  several  of  the  most  important  ores,  particularly  those  of 
lead,  copper  and  iron.  The  most  abundant  sulphate  is  gypsum, 
which  contains  twenty-six  per  cent,  of  sulphur.  In  small  quan- 
tities it  is  widely  diffused  in  rocks,  and  in  the  waters  of  the  ocean. 

Chlorine  is  found  principally  as  an  ingredient  of  rock-salt,  which 
contains  sixty  per  cent,  of  it,  and  of  sea-water,  which  contains  one 
and  a  half  per  cent. 

Fluorine  is  found,  though  very  sparingly,  in  nearly  all  the 
unstratified  rocks.  It  forms  nearly  half  of  the  mineral  known  as 
Derbyshire  spar. 

Of  the  metals,  Iron  is  the  only  one  that  is  found  abundantly. 
It  enters  into  the  composition  of  nearly  all  mineral  substances.  It 
is  generally  combined  with  oxygen,  and  occurs  less  frequently  as  a 
carbonate  or  sulphuret.  Of  volcanic  rocks  it  forms  about  twenty 
per  cent.  Its  ores  are  sometimes  found  in  the  form  of  dikes  or 
seams,  having  been  injected  from  below ;  at  other  times,  in  the 
form  of  nodules  or  stratified  masses,  like  other  rocks  of  mechanical 
origin. 

Manganese  is  likewise  extensively  diffused,  but  in  very  small 
quantity.  The  other  metals  are  often  met  with,  but  their  locali- 
ties are  of  very  limited  extent. 

Of  the  metallic  bases  of  the  earths  and  alkalies,  Silicium  is  the 
most  abundant.  It  generally  occurs  in  the  form  of  silex,  which  is 
an  oxide  of  the  metal.  There  are  but  few  rocks  in  which  it  is 
not  found  in  considerable  amount. 

Aluminium  generally  occurs  as  an  oxide,  in  which  form  it  is 
alumina.  It  is  the  base  of  the  different  varieties  of  clay  and  clay- 
slate.  It  is  also  a  constituent  of  felspar  and  mica. 

Potassium  is  an  ingredient  of  felspar  and  mica,  and  hence  is 


SIMPLE   MINERALS.  13 

found  in  all  the  primary  and  in  most  of  the  volcanic  rocks,  as 
w  ;11  i.s  in  the  stratified  rocks  derived  from  them. 

Sodium  is  a  constituent  of  a  variety  of  felspar  which  is  some- 
what abundant  in  volcanic  rocks.  Its  principal  source  is  the 
extensive  beds  of  rock-salt,  and  the  same  substance  in  a  state  of 
solution  in  the  waters  of  the  ocean. 

Calcium  constitutes  about  forty  per  cent,  of  limestone,  and  is 
an  ingredient  in  nearly  all  igneous  rocks.  This  metal,  in  the  state 
of  an  oxide,  is  lime. 

Magnesium  is  somewhat  abundant,  but  less  so  than  calcium.  It 
is  one  of  the  bases  of  dolomite  and  magnesian  limestone,  and  is  an 
ingredient  of  talc  and  all  talcose  rocks. 

The  substances  now  enumerated  constitute  nearly  the  entire 
mineral  mass  of  the  crust  of  the  earth.  They  may  be  arranged  in 
the  following  order :  — 

I.  NON-METALLIC  SUBSTANCES. 
OXYGEN.  HYDROGEN.  NITROGEN. 

CARBON.  SULPHUR.  CHLORINE. 

FLUORINE. 

II.  METALS. 
IRON.         MANGANESE. 

HI.  METALLIC  BASES  OF  THE  EARTHS  AND   ALKALIES. 
SILICIUM.  ALUMINIUM.  POTASSIUM. 

SODIUM.  CALCIUM.  MAGNESIUM. 

These  substances,  chemically  combined,  form  Simple  Minerals. 


SECTION   II. SIMPLE   MINERALS. 

All  substances  found  in  the  earth  or  upon  its  surface,  which  are 
not  the  products  of  art  or  of  organic  life,  are  regarded  by  the 
mineralogist  as  simple  minerals.  About  four  hundred  mineral 
species  are  known,  and  the  varieties  are  much  more  numerous ;  but 
only  a  small  number  of  them  are  so  abundant  as  to  claim  the 
2 


14  SIMPLE   MINERALS. 

attention  of  the  geologist.  An  acquaintance  with  the  following 
species  is,  however,  necessary. 

Quartz  is  probably  the  most  abundant  mineral  in  nature.  It 
is  composed  wholly  of  silex.  Its  specific  gravity  is  2.65.  It  is  the 
hardest  of  the  common  minerals,  gives  sparks  with  steel,  scratches 
glass,  and  breaks  into  irregular  angular  fragments  under  the  hammer. 
When  crystallized,  its  most  common  form  is  that  of  a  six-sided 
prism,  terminated  by  six-sided  pyramids.  When  pure,  it  is  trans- 
parent or  translucent,  and  its  lustre  is  highly  vitreous.  The 
transparent  variety  is  called  rock  crystal.  When  purple,  it  is 
amethyst.  When  faint  red,  it  is  rose  quartz.  When  its  color  is 
dark  brown,  or  gray,  and  it  has  a  conchoidal  fracture,  it  is  flint. 
When  quartz  occurs  in  white,  tuberous  masses,  of  a  resinous  lustre 
and  conchoidal  fracture,  it  is  opal.  The  precious  opal  is  distin- 
guished by  its  lively  play  of  colors.  Jasper  is  opaque,  and  con- 
tains a  small  per  cent,  of  oxide  of  iron,  by  which  it  is  colored  dull 
red,  yellowish  red  or  brown.  The  light-colored,  massive,  trans- 
lucent variety  is  chalcedony.  The  flesh-colored  specimens  are 
carnelian.  When  composed  of  layers  of  chalcedony  of  different 
colors,  it  becomes  agate.  Several  of  the  varieties  of  quartz,  such 
as  amethyst,  opal,  carnelian  and  agate,  are  used  to  considerable 
extent  in  jewelry. 

Felspar  is  composed  of  silex,  alumina  and  potassa.  It  resembles 
quartz,  but  it  is  not  as  hard,  cleaves  more  readily,  and  is  not  gen- 
erally transparent.  Its  specific  gravity  is  2.47.  Its  lustre  is 
feebly  vitreous,  but  pearly  on  its  cleavage  faces.  Its  color  is 
sometimes  green,  but  generally  dull  white,  and  often  inclined  to 
red  or  flesh-color. 

Mica  is  composed  of  the  same  ingredients  as  felspar,  together 
with  oxide  of  iron.  Its  specific  gravity  is  nearly  three.  It  is 
often  colorless,  but  frequently  green,  smoky,  or  black.  It  may 
be  known  by  its  capability  of  division  into  exceedingly  thin,  trans- 
parent, elastic  plates. 

Hornblende  is  composed  of  silex,  alumina  and  magnesia.  Its 
specific  gravity  is  a  little  above  three.  Its  color  is  generally  some 


SIMPLE   MINERALS.  15 

shade  of  green.  When  dark  green  or  black,  whether  in  a  massive 
or  crystalline  state,  it  is  common  hornblende.  When  light  green, 
it  is  actinolite.  •  The  white  variety  is  tremolite.  When  it  is  com- 
posed of  flexible  fibres,  it  is  asbestus  ;  and  when  the  fibres  have  also 
a  silky  lustre,  it  is  amianthus. 

Augite  or  Pyroxene  has,  till  recently,  been  considered  as  a 
variety  of  hornblende.  Its  specific  gravity  is  slightly  different ; 
its  composition  is  the  same,  and  in  general  appearance  it  is  not 
easily  distinguished  from  hornblende.  It  has,  however,  been  made 
a  distinct  species,  because  its  crystalline  form  is  different. 

Hypersthene  is  composed  of  silex,  magnesia  and  oxide  of  iron. 
Its  specific  gravity  is  3.38.  It  closely  resembles  hornblende.  The 
lustre  of  its  cleavage  faces  is  metallic  pearly.  Its  color  is  grayish 
or  greenish  black. 

Talc  is  composed  of  silex  and  magnesia.  Its  specific  gravity  is 
2.7.  It  resembles  mica  in  its  general  appearance  and  in  its  lam- 
ellar structure,  but  it  is  easily  distinguished  from  it  by  its  plates 
being  not  elastic,  and  by  its  soapy  feel.  Its  color  is  generally  some 
shade  of  green.  Soapstone  is  an  impure  variety  of  talc,  of  a  light 
gray  color,  earthy  texture,  and  is  unctuous  to  the  touch.  Chlo- 
rite, another  impure  variety,  is  a  dark  green  rock,  massive,  easily 
cut  with  a  knife,  and  unctuous  to  the  touch. 

Serpentine  is  composed  of  silex  and  magnesia.  Its  specific 
gravity  is  2.55.  It  is  generally  massive,  unctuous  to  the 
touch,  and  of  a  green  color.  It  is  often  variegated  with  spots 
of  green  of  different  shades.  With  a  mixture  of  carbonate  of 
lime  it  forms  the  verd  antique  marble. 

Carbonate  of  Lime,  or  common  limestone,  is  composed  of  car- 
bonic acid  and  lime.  Its  specific  gravity  is  2.65.  It  presents  a 
great  variety  of  forms.  In  a  crystalline  state  it  is  generally  trans- 
parent, and  when  so,  possesses  the  property  of  double  refraction. 
It  may  be  distinguished  from  every  other  common  species  by  its 
rapid  effervescence  with  acids.  It  readily  cleaves  parallel  to  all 
the  faces  of  the  primary  form,  which  is  a  rhombohedron. 

Sulphate  of  Lime,  or  Gypsum,  is  composed  of  sulphuric  acid 


16  MINERAL   MASSES. 

and  lime.  Its  specific  gravity  is  2.32.  When  crystalline,  it  has  a 
pearly  lustre,  is  transparent,  and  goes  under  the  name  of  Selenite. 
Common  Gypsum  resembles  the  other  earthy  limestones,  but  it  is 
softer,  and  may  be  readily  distinguished  by  its  not  effervescing 
with  acids. 

To  the  minerals  now  enumerated  may  be  added  the  following, 
which  are  of  frequent  occurrence,  but  not  in  great  quantities ; 
namely,  carbonate  of  magnesia,  oxide  of  iron,  iron  pyrites,  rock- 
salt,  coal,  bitumen,  schorl  and  garnet. 

These  simple  minerals,  either  in  separate  masses  or  mingled 
more  or  less  intimately  together,  compose  almost  wholly  the 
earth's  crust. 


SECTION   III. THE   MINERAL   MASSES   WHICH    FORM    THE   CRUST   OF 

THE   EARTH. 

That  portion  of  the  structure  of  the  earth  which  is  accessible  to 
man  is  called  the  crust  of  the  earth. 

The  mineral  masses  which  compose  it,  whether  in  a  solid  state, 
like  granite  and  limestone,  or  in  a  yielding  state,  like  beds  of  sand 
and  clay,  are  called  rocks. 

The  unstratified  rocks  are  Granite,  Hypersthene  rock,  Lime- 
stone and  Serpentine,  and  the  Trappean  and  Volcanic  rocks. 

Granite  is  a  rock  of  a  light  gray  color,  and  is  composed  of 
quartz,  felspar  and  mica,  in  variable  proportions,  confusedly  crys- 
tallized together.  The  felspar  is  generally  the  predominant  min- 
eral. It  is  sometimes  of  a  very  coarse  texture,  the  separate  min- 
erals occurring  in  masses  of  a  foot  or  more  in  diameter.  At  other 
times  it  is  so  fine-grained  that  the  constituent  minerals  can  scarcely 
be  recognized  by  the  naked  eye ;  and  between  these  extremes  there 
is  every  variety.  The  term  granite  is  not,  however,  confined  to  an 
aggregate  of  these  three  minerals.  In  some  instances  the  felspar 
so  predominates  as  almost  to  exclude  the  other  minerals,  when  it 
is  called  felspathic  granite.  When  the  quartz  appears  in  the 
form  of  irregular  and  broken  lines,  somewhat  resembling  written 


illNEKAL    MASSES.  17 

characters,  in  a  base  of  felspar,  it  is  called  graphic  granite.  When 
talc  takes  the  place  of  mica,  it  is  talcose  granite.  When  horn- 
blende takes  the  place  of  mica,  it  is  syenite.  Granite  or  any 
rock  becomes  porphyritic  when  it  contains  imbedded  crystals  of 
felspar. 

There  is  a  rock  of  crystalline  structure,  like  granite,  but  of  a 
darker  color,  which  is  called  hypersthene  rock.  It  is  composed  of 
Labrador  felspar  and  hypersthene.  The  mineral  species  serpen- 
tine and  limestone  often  occur  unstratified  in  considerable  quantities. 

Volcanic  rocks  consist  of  the  materials  ejected  from  the  cra- 
ters of  volcanoes.  They  are  composed  of  essentially  the  same  min- 
erals as  trap  rocks.  When  the  material  has  been  thrown  out  in  a 
melted  state,  it  is  called  lava.  Lava,  at  the  time  of  its  ejection, 
contains  a  large  amount  of  watery  vapor  at  a  high  temperature. 
Under  the  immense  pressure  to  which  it  is  subjected  in  the  volcanic 
foci,  it  may  exist  in  the  form  of  water  ;  but  when  the  lava  is  thrown 
out  at  the  crater,  the  pressure  cannot  much  exceed  that  of  the 
atmosphere.  The  particles  of  water  at  once  assume  the  gaseous 
form.  As  lava  possesses  considerable  viscidity,  the  steam  does  not 
escape,  but  renders  the  upper  portion  of  the  mass  vesicular.  This 
vesicular  lava  is  called  scorice.  By  the  movement  of  the  stream 
of  lava,  these  vesicles  become  drawn  out  into  fine  capillary  tubes, 
converting  the  scoriae  into  pumice-stone. 

A  large  part  of  the  materials  ejected  from  volcanoes  is  in  the 
form  of  dust,  cinders  and  angular  fragments  of  rock.  These  soon 
become  solidified,  forming  volcanic  tuff,  or  volcanic  breccia.  In 
submarine  eruptions  these  fragments  are  spread  out  by  the  water 
into  strata,  upon  which  other  materials,  not  volcanic,  are  after- 
wards deposited.  These  interposed  strata  are  called  volcanic  grits. 

The  trappean  rocks  are  composed  of  felspar,  mingled  intimately 
and  in  small  particles  with  augite  or  hornblende.  They  also  con- 
tain iron  and  potassa.  They  are  often  porphyritic.  When  they 
contain  spherical  cavities,  filled  with  some  other  mineral,  such  as 
chlorite,  carbonate  of  lime  or  agate,  they  are  called  amygdaloidal 
trap. 

2* 


18 


MINERAL    MASSES. 


The  principal  varieties  of  trappean  rock  are  basalt,  green  stone, 
and  trachyte.  In  basalt,  augite,  or,  in  some  cases,  hornblende,  is 
the  predominant  mineral.  It  is  a  heavy,  close-grained  rock,  of  a 
black  or  dark  brown  color.  Greenstone  differs  from  basalt  in 
containing  a  much  larger  proportion  of  felspar.  Its  structure  is 
more  granular,  and  frequently  it  assumes  so  much  of  the  crystal- 
line form  as  to  pass  insensibly  into  syenite  or  granite.  It  is  a 
dark  colored  rock,  with  a  slight  tinge  of  green.  Both  green  stone 
and  basalt  are  disposed  to  assume  the  columnar  form,  the  columns 
being  arranged  at  right  angles  to  the  faces  of  the  fissure  into  which 
the  trap  is  injected.  When  it  is  spread  out  into  broad  horizontal 


Pig.  1 


masses,  the  columns  are  vertical.  (Fig.  1.)   Trachyte  is  composed 
principally  of  felspar,  is  of  a  grayish  color,  and  rough  to  the  touch. 
Of  the  stratified  rocks  the  following  are  the  most  important : 
Gneiss  is  a  rock  closely  resembling  granite.     It  is  an  aggregate 
of  the  same  minerals,  but  the  proportion  of  mica   is  somewhat 
greater.     The  only  distinction  between  them  is  that  the  gneiss  is 
stratified,  but  the  stratification  is  often  so  indistinct  that  it  passes 
insensibly  into  granite.     Generally,  however,  the  stratification  is 
so  distinct  as  to  present  a  marked  difference. 

Mica   slate   is    such  a  modification  of  mieiss   that    the  mica 


MINERAL   MASSES.  19 

becomes  the  predominant  mineral,  with  a  small  intermixture  of 
quartz  and  felspar.  Consequently  the  stratification  becomes  very 
distinct,  so  as  sometimes  to  render  the  mass  divisible  into  thin 
sheets.  The  stratification  is  often  wavy,  and  sometimes  much 
contorted. 

Sandstone  consists  of  grains  or  fragments  of  any  other  rock, 
but  more  frequently  of  siliceous  rocks.  The  fragments  are  con- 
solidated, sometimes  without  any  visible  cement,  but  often  by  a 
paste  of  argillaceous  or  calcareous  substance.  The  color  varies 
with  that  of  the  rock  from  which  it  was  derived.  Generally,  how- 
ever, it  is  either  drab  or  is  colored  red  by  oxide  of  iron.  The 
fragments  are  sometimes  so  minute  as  scarcely  to  give  the  rock  the 
appearance  of  sandstone.  When  they  are  of  considerable  size  and 
rounded,  the  rock  is  called  conglomerate.  When  they  are  angu- 
lar, it  is  called  breccia.  Greensand  is  a  friable  mixture  of  sili- 
ceous and  calcareous  particles,  colored  by  a  slight  intermixture  of 
green  earth  or  chlorite. 

Limestone  is  a  very  abundant  rock,  and  occurs  in  many  differ- 
ent forms.  In  transparent  crystals  it  is  Iceland  spar.  When 
white  and  crystalline,  it  is  primary  limestone,  saccharine  lime- 
stone, or  statuary  marble.  When  sub-crystalline  it  is  generally 
more  or  less  colored.  It  is  often  clouded  with  bands  or  patches  of 
white  in  a  ground  of  some  dark  color.  When  its  texture  is  close, 
and  the  crystallization  scarcely  apparent,  it  is  compact  limestone. 
The  white,  earthy  variety  is  chalk.  A  variety  of  limestone  com- 
posed of  small  spheres  is  called  oolite.  Lias  is  the  name  given 
to  an  impure  argillaceous  variety  of  a  brown  or  blue  color.  Any 
rock  which  contains  a  considerable  proportion  of  carbonate  of  lime, 
and  which  rapidly  disintegrates  on  exposure  to  the  atmosphere,  is 
called  marl.  Limestone  sometimes  contains  carbonate  of  magnesia. 
It  is  then  magnesian  limestone,  or  dolomite. 

Clay  consists  of  a  mixture  of  siliceous  and  aluminous  earth.  It 
is  tough,  highly  plastic,  and  generally  of  a  lead  blue  color.  It  is 
always  stratified,  and  often  divided  into  very  thin  laminae,  which 


20  MINERAL   MASSES. 

are  separated  by  sprinklings  of  sand  only  sufficient  to  keep  them 
distinct. 

Clay  slate,  or  argillaceous  schist,  is  composed  of  the  same  mate- 
rials as  clay,  and  differs  from  it  only  in  having  become  solidified. 
Its  color  is  gray,  dark  brown  or  black.  In  some  beds  it  is  purple. 
Shale  is  the  same  material  in  a  state  of  partial  solidification.  On 
exposure  to  the  weather,  it  soon  disintegrates,  and  is  finally  recon- 
verted into  clay.  All  the  varieties  of  argillaceous  rock  are  easily 
distinguished  by  a  peculiar  odor  which  they  emit  when  breathed 
upon. 

Argillaceous  slate  sometimes  takas  into  its  composition  portions 
of  some  other  mineral,  such  as  talc,  mica,  or  hornblende.  When 
any  of  these  minerals  becomes  so  abundant  as  to  constitute  a  con- 
siderable part  of  the  mass,  the  rock  becomes  talcose,  micaceous,  or 
hornblende  slate.  Sometimes  this  last  variety  loses  all  appear- 
ance of  a  fissile  structure,  and  is  composed  almost  wholly  of  horn- 
blende. It  is  then  called  hornblende  rock. 

Diluvium  is  the  name  applied  to  masses  of  sand,  gravel,  and 
large  rocks,  called  boulders,  heaped  confusedly  together  on  the 
surface  of  the  earth.  It  is  also  called  drift. 


-    CHAPTER    II. 

OF    THE   ARRANGEMENT  OF    THE  MATERIALS  WHICH   COM- 
POSE THE  'CRUST  OF  THE  EARTH. 

SECTION   I. THE   CLASSIFICATION   OF   ROCKS. 

IN  the  first  place,  we  divide  rocks  into  stratified  and  un- 
stratified.  This  division  is  one  which  will  in  general  be  easily 
recognized,  even  by  the  most  inexperienced  observer ;  and  the  dis- 
tinction is  important,  because  it  separates  the  rocks  of  igneous  ori- 
gin from  those  which  have  been  produced  by  deposition  of  sedi- 
ment from  water. 

It  will  be  shown  hereafter  that  a  part  of  the  unstratified  rocks 
have  been  formed  at  or  near  the  surface  of  the  earth ;  that  is,  they 
have  taken  their  present  form  by  passing  from  a  state  of  fusion  to 
a  solid  state  above  or  between  the  stratified  rocks,  as  in  the  case  of 
lava  (Fig.  2,  A).  The  other  unstratified  rocks  have  cooled  so  as 
to  take  the  solid  form  below  the  stratified  rocks,  as  at  B.  The 
first  are  called  epigerie,  or  volcanic  rocks  ;  the  last,  hypogene,  or 
plutonic  rocks. 

The  lowest  portion  of  the  second  division,  the  stratified  rocks, 
are  termed  non-fossiliferous,  from  the  fact  that  they  contain  no 
evidence  of  the  existence  of  organic  beings  at  the  time  when  they 
were  deposited.  Their  relation  to  the  other  rocks  is  shown  at  C. 
It  is  supposed -that  these  rocks  have  been  subjected  to  great 
changes  by  heat  from  the  igneous  rocks  below  them.  On  this 
account  Mr.  Lyell  proposes  to  call  them  metamorphic  rocks.  The 
other  portions  of  the  stratified  rocks  are  fossiliferous,  containing 
the  remains  of  organic  beings  which  lived  at  the  period  when  the 


CLASSIFICATION    OF    ROCKS. 


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!;j  1^ \""^""^  ^  "•"  *"  ^  — -  ~ ~~^--"~' " ^ '  r">;  -^i-1^ 
I  ilj'  •  \  ^i"^-^  ^'-'^  <'^ij  L  ^-J  '•  s-^x^ 

r   vTiiif^&?-->£^ 

*  f»®^ 

lv\\t,//  sars.V -i/1;- 


PLUTONIC   ROCKS.  23 

rocks  were  deposited.     They  are  represented  at  D.     The  division 
of  the  last-named  rocks  info  groups  will  be  given  hereafter. 

We  have  then  four  principal  classes  of  rocks :  Plutonic 
Rocks,  Volcanic  Rocks,  Non-fossUiferous  Strati/ied  Rocks  and 
Fossiliferous  Rocks. 


SECTION   II.  —  THE   PLUTONIC   ROCKS. 

Granite  is  by  far  the  most  important  of  this  class  of  rocks.  Of 
its  thickness  no  estimate  can  be  made,  as  no  mining  operations 
have  ever  penetrated  through  it,  and  none  of  the  most  extensive 
displacements  of  rocks  by  natural  causes  has  brought  to  the  sur- 
face any  other  rock  on  which  it  rests.  It  may,  therefore,  be  con- 
sidered the  foundation  rock,  the  skeleton  of  the  earth,  upon  which 
all  the  other  formations  are  supported.  The  whole  amount  of 
granite  in  the  earth's  crust  may  be  greater  than  that  of  all  other 
rocks,  but  it  comes  up  through  the  other  formations  so  as  to  be 
exposed  over  only  a  comparatively  small  portion  of  the  surface, 
and  this  is  generally  the  central  portion  of  mountain  ranges,  or  the 
highest  parts  of  broken,  hill  country.  Still,  it  is  not  unfrequently 
found  in  the  more  level  regions,  in  the  form  of  slightly  elevated 
ridges,  with  the  stratified  rocks  reclining  against  it. 

The  structure  of  granite  seems  frequently  to  be  a  confused 
mixture  of  the  minerals  which  compose  it,  without  any  approach 
to  order  in  their  arrangement ;  but  in  many  cases  it  is  found  to 
split  freely  in  certain  directions,  and  to  work  with  difficulty  in  any 
other.  This  may  result  from  an  arrangement  of  the  integrant 
crystals,  so  that  their  cleavage  planes  approach  more  or  less  nearly 
to  parallelism.  When  this  is  the  case  with  the  mica  or  felspar,  it 
must  diminish  the  cohesion  in  a  direction  perpendicular  to  these 
planes,  and  thus  facilitate  the  cleavage  of  the  mass. 

Granite  is  found  to  penetrate  the  stratified  rocks  in  the  form  of 
veins.  The  following  section  (Fig.  8)  will  show  the  relation  of 


24 


PLUTONIC    ROCKS. 


Fig.  3. 


granite  veins  to  the  granitic  mass  below.  The  granite  which  is 
quarried  for  architectural  purposes  is  often  in  comparatively  small 
quantities,  disappearing  at  the  distance 
of  a  few  hundred  yards  beneath  the 
stratified  rock ;  or  else  it  exists  in  the 
form  of  isolated  dome-shaped  masses. 
It  is  probable  that,  if  they  could  be  fol- 
lowed sufficiently  far,  they  would  be 
found  to  be  portions  of  dikes  coming 
from  the  general  mass  of  granite  be- 
low. Even  the  granite  nuclei  of  the 
great  mountain  ranges  may  be  consid- 
ered as  injected  dikes  of  enormous  mag- 
.nitude. 


Fig.  4. 


Granite  is  itself  intersected  with  granite  veins  more  frequently, 
perhaps,  than  any  other  rocks ;  but  the  vein  is  a  coarser  granite 

than  the  rock  which  it  divides. 
It  is  not  uncommon  to  find  one  set 
of  dikes  intercepted  and  cut  off  by 
a  second  set,  and  the  second  by  a 
third.  The  substance  of  the  dikes 
was,  of  course,  in  a  liquid  state  when 
it  was  injected,  and  the  first  must 
have  become  solid  before  the  second 
was  thrown  in  ;  hence  the  dikes  are 
of  different  ages.  The  dikes  a  b  c, 
represented  in  Fig.  4,  must  have 
been  injected  in  the  order  in  which  they  are  lettered. 

It  is  probable  that,  by  the  process  of  cooling,  the  liquid  mass 
from  which  these  dikes  have  proceeded  has  been  gradually  solid- 
ifying from  the  surface  downwards.  If  so,  it  would  follow  that 
the  granite  nearest  the  surface  (1,  Fig.  2)  is  the  oldest,  and  the 
newest  is  that  which  is  at  the  greatest  distance  below  (4).  It  is 
possible  that  at  great  depths  granite  may  be  still  forming,  —  that 
is,  taking  the  solid  form,  —  though  of  this  there  can  be  no  direct 


VOLCANIC   ROCKS.  25 

proof.  There  is,  however,  proof  that  it  has  been  liquid  at  periods 
of  time  very  distant  from  each  other ;  for  the  dikes  sometimes 
reach  to  the  top  of  the  coal  formation  (for  example),  and  then 
spread  themselves  out  horizontally,  as  at  a,  showing  that  the  rock 
above  the  coal  had  not  then  been  deposited.  Another  dike  will 
extend  through  the  new  red  sandstone,  as  at  b,  and  spread  itself  out 
horizontally  as  before.  These  horizontal  layers  of  granite,  by  their 
position  in  strata  whose  ages  are  known,  indicate  the  periods  when 
granite  has  existed  in  a  liquid  state.  Granite  veins  have  been  dis- 
covered in  the  Pyrenees  as  recent  as  the  close  of  the  cretaceous 
period,  and  in  the  Andes  they  have  been  found  among  the  tertiary 
rocks. 

There  are  several  other  rocks,  of  minor  importance,  often  found 
in  connection  with  granite.  Hypersthene  rock,  in  a  few  cases, 
forms  the  principal  part  of  mountain  masses.  Greenstone  is  more 
frequently  associated  with  the  trappean  rocks,  but  it  sometimes 
passes  imperceptibly  into  syenite  and  common  granite.  Limestone 
is  found  in  considerable  abundance,  and  serpentine  in  small  quan- 
tities, as  primary  rocks,  and  have  evidently  been  formed  like 
granite,  by  solidiiying  from  a  state  of  fusion. 


SECTION   ni. THE   VOLCANIC   BOCKS. 

The  volcanic  rocks  consist  of  materials  ejected  from  volcanoes. 
They  are,  however,  ejected  in  very  different  states  ;  sometimes  as 
dust,  sand,  angular  fragments  of  rock,  cinders,  &c.,  and  sometimes 
as  lava  streams.  In  some  instances,  the  lava  has  so  little  fluidity 
that  it  accumulates  in  a  dome-shaped  mass  over  the  orifice  of  erup- 
tion, and  perhaps  in  a  few  instances  it  has  been  thrust  upward  in 
a  solid  state. 

There  are  two  principal  varieties  of  lava,  —  the  trachytic,  con- 
sisting mostly  of  felspar,  and  the  basaltic,  consisting  of  hornblende. 
When  both  kinds  are  products  of  the  same  eruption,  the  trachytic 
lava  is  thrown  out  first,  and  the  basaltic  last.  The  reason  of 
3 


26  VOLCANIC    KOCKS. 

this  is,  that  felspar  is  lighter  than  hornblende,  and  probably  rises 
to  the  surface  of  the  lava  mass  at  the  volcanic  focus,  and  the  basal- 
tic lava  is  therefore  reserved  till  the  trachytic  has  been  thrown  off. 

These,  like  other  rocks,  have  been  produced  at  different  epochs. 
There  is,  however,  great  difficulty  in  determining  their  age;  There 
are  some  differences  of  structure  and  composition  observed,  in  com- 
paring the  older  and  newer  lavas ;  but  the  only  method  that  can  be 
relied  on  to  determine  their  age  is  their  relation  to  other  rocks. 
When  they  occur  between  strata  whose  age  is  determined  by  imbed- 
ded fossils,  they  must  be  of  intermediate  age  between  the  inferior 
and  superior  strata. 

1.  Modern  Volcanic  Rocks.  —  Some  of  the  volcanic  rocks  are 
of  modern  origin,  and  are  produced  by  volcanoes  now  active.  The 
total  amount  of  these,  and  of  all  the  other  volcanic  rocks,  is  prob- 
ably less  than  that  of  either  of  the  other  principal  divisions  of 
rocks  ;  yet  they  form  no  inconsiderable  part  of  the  earth's  crust. 
The  number  of  active  volcanoes  is  not  far  from  three  hundred,  and 
the  number  of  eruptions  annually  is  estimated  at  about  twenty. 
In  some  cases,  the  la"va  consists  of  only  a  single  stream,  of  but  a 
few  hundred  yards  in  extent.  It  extends,  however,  not  unfre- 
quently  twenty  miles  in  length,  and  two  or  three  hundred  yards  in 
breadth.  The  eruption  of  Mount  Loa,  on  the  island  of  Hawaii,  in 
1840,  from  the  crater  of  Kilauea,  covered  an  area  of  fifteen  square 
miles  to  the  depth  of  twelve  feet ;  and  another  eruption  of  the  same 
mountain,  in  1843,  covered  an  area  of  at  least  fifty  square  miles. 
The  eruption  in  Iceland,  in  1783,  continued  in  almost  incessant 
activity  for  a  year,  and  sent  off  two  streams  in  .opposite  directions, 
which  reached  a  distance  of  fifty  miles  in  one  case,  and  of  forty  in 
the  other,  with  a  width  varying  from  three  to  fifteen  miles,  and  with 
an  average  depth  of  more  than  a  hundred  feet.  The  size  of  some 
of  the  volcanic  mountains  will  also  assist  in  forming  an  idea  of  the 
amount  of  volcanic  rocks.  Monte  Nuovo,  near  Naples,  which  is  a 
mile  and  a  half  in  circumference  and  four  hundred  and  forty  feet 
high,  was  thrown  up  in  a  single  day.  ^Etna,  which  is  eleven  thou- 
sand feet  high,  and  eighty-seven  miles  in  circumference  at  its 


VOLCANIC   ROCKS.  27 

base,  has  probably  been  produced  wholly  by  its  own  eruptions.  A 
large  part  of  the  chain  of  the  Andes  consists  of  volcanic  rock,  but 
the  proportion  we  have  not  the  means  of  estimating. 

2.  Tertiary  Lavas.  —  There  is  another  class  of  volcanic  prod- 
ucts, which  are  so  situated  with  reference  to  the  tertiary  strata 
that  they  must  be  referred  to  that  period.  The  principal  locali- 
ties of  these  lavas,  so  far  as  yet  known,  are  Italy,  Spain,  Central 
France,  Hungary,  and  Germany.  They  are  also  found  in  South 
America.  Those  of  Central  France  have  been  studied  with  the 
most  care.  They  occur  in  several  groups,  but  they  were  the  seats 
of  volcanic  activity  during  the  same  epoch,  and  formed  parts  of  one 
extensive  volcanic  region.  Each  of  these  minor  areas,  embracing 
a  circle  of  twenty  or  thirty  miles  in  diameter,  is  covered  with  hills 
two  or  three  thousand  feet  in  height,  which  are  composed  entirely 
of  volcanic  products,  like  the  cone  of  JEtna.  On  many  of  them 
there  are  perfectly-formed  craters  still  remaining.  Numerous 
streams  of  lava  have  flowed  from  these  craters,  some  of  which  can 
now  be  traced,  throughout  their  whole  extent,  with  as  much  cer- 
tainty as  if  they  were  eruptions  of  the  present  century.  Some  of 
the  lavas  have  accumulated  around  the  orifices  of  eruption,  form- 
ing rounded,  dome-shaped  eminences.  These  lavas  generally  con- 
sist of  trachyte,  and  have  therefore  a  low  specific  gravity,  and 
imperfect  fluidity.  The  basaltic  lavas  have  often  spread  out  over 
broad  areas,  and,  when  they  have  been  confined  in  valleys,  have 
reached  a  distance  of  fifteen  miles  or  more  from  their  source. 
There  still  remain  indications  of  a  current  of  lava/  which  was 


thirty  miles  long,  six  broad,  and  in  a  part  of  its  course  from  four 
to  six  hundred  feet  deep.     The  above  sketch  (Fig.  5)  will  give 


28  VOLCANIC   ROCKS. 

some  idea  of  the  highly  volcanic  aspect  which  the  district  of 
Auvergne,  in  France,  presents. 

The  unimpaired  state  of  some  of  the  cones  and  craters,  and  of 
the  lava  currents,  would  lead  to  the  impression  that  these  regions 
have  been  the  theatre  of  intense  volcanic  action  within  a  very 
recent  period.  But  there  is  good  reason  to  believe  that  this  has 
not  been  the  case.  "  The  high  antiquity  of  the  most  modern  of 
these  volcanoes  is  indeed  sufficiently  obvious.  Had  any  of  them 
been  in  a  state  of  activity  in  the  age  of  Julius  Caesar,  that  general, 
who  encamped  upon  the  plains  of  Auvergne  and  laid  siege  to  its 
principal  city,  could  hardly  have  failed  to  notice  them." 

It  is  equally  certain  that  the  commencement  of  their  activity 
was  at  a  late  period  in  the  history  of  the  earth.  Lava  currents 
are  frequently  found  in  France  resting  upon  the  early  tertiary 
strata,  but  no  lava  current  is  found  below  them.  The  later  ter- 
tiary strata  contain  pebbles  of  volcanic  rocks,  showing  that  lavas 
had  been  previously  ejected,  but  none  are  found  in  the  older  strata 
of  this  formation.  We  must,  therefore,  conclude  that  these  vol- 
canic tracts  assumed  their  volcanic  character  at  some  intermediate 
point  in  the  tertiary  period. 

When  we  find  that  their  activity  commenced  at  so  late  a  period 
and  closed  so  long  ago,  we  might  be  led  to  suppose  that  it  was  of 
very  short  duration.  But  a  great  number  of  facts,  in  the  present 
condition  of  the  country,  require  that  we  should  assign  to  them  a 
very  prolonged  activity.  A  single  instance  will  be  sufficient  to 
show  the  nature  of  the  evidence  upon  which  this  conclusion  rests. 
The  heavy  line  (Fig.  6)  represents  the  present  form  of  one  of  the 
valleys.  A  bed  of  lava  forms  the  highest  point  of  land  repre- 
sented, and  a  second  bed  is  found  in  an  intermediate  part  of  the 
slope.  The  position  of  the  upper  bed  must  have  been  a  valley, 
when  the  lava  flowed  there.  We  may  represent  this  valley  by  the 
line  a  b  c.  The  slow  operation  of  natural  denuding  causes  at 
length  excavated  the  valley  d  e  h,  when  another  lava  current 
flowed  through  it,  covering  its  bed  of  pebbles,  as  before.  The  same 
denuding  causes  have  at  length  produced  the  present  valley,  /  g  h. 


\OLCAXIC    ROCKS. 


29 


These  remnants  of  lava-currents,  as  they  have  formed  a  very  imper- 
ishable rock,  have  protected  the  subjacent  strata  from  erosion,  and 

Fig.  6. 


furnish  evidence  of  the  position  of  the  valley  at  different  periods. 
When  we  consider  with  what  extreme  slowness  denuding  causes 
produce  changes  on  the  surface,  and  what  extensive  changes  they 
have  here  nevertheless  effected  in  the  interval  between  the  produc- 
tion of  the  different  lava  currents,  we  are  compelled  to  feel  that 
that  interval  was  a  very  prolonged  one.  Yet  this  period,  however 
long  it  may  have  been,  was  evidently  less  than'the  period  of  activ- 
ity of  these  volcanoes. 

3.  Volcanic  rocks  of  an  earlier  date  are  also  found,  sometimes 
as  distinct  lavas,  though  generally  as  volcanic  grits.  They  occur 
inter-stratified  with  the  cretaceous  rocks,  and  with  every  other 
formation  of  the  fossiliferous  series,  showing  that,  from  the  earliest 
times,  these  rocks  have  been  accumulating  as  they  now  are. 

The  trappean  rocks  may,  in  a  general  classification,  be  consid- 
ered as  volcanic.  It  will  be  shown,  hereafter,  that  they  are  the 
lavas  of  submarine  volcanoes.  They  do  not,  however,  occur  in  the 
form  of  lava  currents,  but  in  great  tabular  masses,  generally 
between  stratified  rocks,  or  in  the  form  of  dikes.  They  are  also 
entirely  unconnected  with  cones  or  craters. 

The  trappean  rocks  occur  more  or  less  abundantly  in  all  coun- 
tries. One  of  the  most  noted  localities  of  this  rock  is  a  region 
3* 


30  NON-FOSSILIFEROUS   STRATIFIED   ROCKS. 

embracing  the  north  of  Ireland,  arid  several  of  the  islands  on  the 
western  coast  of  Scotland.  It  contains  the  celebrated  Giant's 
Causeway,  which  consists  of  a  mass  of  columnar  trap  ;  also  Fingal's 
Cave,  which  is  produced  by  a  portion  of  the  trap  being  columnar, 
and  thus  disintegrating  more  rapidly  than  the  rest,  by  the  action 
of  the  waves.  An  immense  mass  of  greenstone  trap,  which  has 
generally  been  considered  as  a  vast  dike,  though  often  a  mile  in 
thickness,  is  found  extending  from  New  Haven  to  Northampton, 
on  the  west  side  of  the  Connecticut  river.  It  then  crosses  to  the 
east  side,  and  continues  in  a  northerly  direction  to  the  Massachu- 
setts line.  Under  different  names,  it  constitutes  a  nearly  contin- 
uous and  precipitous  mountain  range  for  about  one  hundred  miles. 
Dr.  Hitchcock  supposes  this  greenstone  range  to  be,  not  an  injected 
dike,  but  a  tabular  mass  of  ancient  lava,  which  was  spread  out  on 
the  bed  of  the  ocean  during  the  period  of  the  deposition  of  the 
Connecticut  river  sandstone.  It  was  subsequently  covered  with  a 
deposit  of  strata  of  great  thickness,  and  then  by  subterranean  forces 
thrown  into  its  present  inclined  position. 

There  is  a  mass  of  basaltic  rock  in  the  valley  of  the  Columbia 
river,  in  the  Oregon  Territory,  which  extends  without  interruption 
for  a  distance  of  foul*  hundred  miles.  Its  breadth  and  thickness  is 
not  known,  but  in  some  places  the  river  has  cut  a  channel  in  this 
rock  to  a  depth  of  four  hundred  feet.  Its  age  has  not  been  deter- 
mined, and  it  will,  perhaps,  be  found  to  be  a  tertiary  or  modern 
production. 


SECTION     IV. THE    NON-FOSSILIFEROUS     STRATIFIED   (OR   METAMOR- 

PHIC)   ROCKS. 

1.  Gneiss  is  the  most  abundant  rock  in  this  class,  and  is  gen- 
erally found  reposing  on  granite.  Its  stratification  is  sometimes 
very  distinct,  but  it  is  often  so  imperfect  that  it  can  scarcely  be 
recognized.  This  is  more  frequently  the  case  in  the  vicinity  of 
granite  on  which  it  rests,  and  into  which  it  insensibly  passes.  A 


NON-FOSSILIFEROUS   STRATIFIED   ROCKS.  31 

large  part  of  the  material  used  for  building  purposes,  under  the 
name  of  granite,  is  obscurely  marked  gneiss.  In  all  primary 
countries  it  is  an  abundant  rock,  occupying  extensive  districts,  and 
sometimes  forming  mountain  masses. 

2.  Mica  slate  lies  next  above  gneiss,  and  is  a  very  abundant 
rock.     As  it  differs  from  gneiss  only  in  the  proportion  of  mica 
which  it  contains,  and  as  the  quantity  of  mica  in  it  is  very  dif- 
ferent in  different  places,  it  is  often  difficult  to  make  the  distinc- 
tion between  them.     It  also  passes  by  insensible  degrees  into  the 
argillaceous  rocks.    Many  of  the  argillaceous  rocks  are  found,  upon 
close  examination,  to  contain  mica  in  minute  scales  in  such  abund- 
ance as  to  make  it  doubtful  whether  they  ought  not  to  be  regarded 
as  mica  slates ;  that  is,  the  metarnorphic  action  by  which  argillaceous 
slate  is  converted  into  mica  slate  had  proceeded  so  far,  before  it  was 
arrested,  that  it  becomes  impossible  to  say  whether  the  argillaceous 
or  micaceous  characters  predominate. 

3.  Argillaceous  slate.  —  The  last  rock  of  this  series  is  a  slaty 
rock,  more  or  less  highly  argillaceous.     It  does  not  differ  in  litho- 
logical  characters  from  the  same  rock  in  the  higher  strata.     It  is 
doubtful  whether  the  roofing-slates  should  be  considered  as  belong- 
ing to  the  metamorphic  series  or  not.     They  have  been  subjected 
to  a  very  high  degree  of  metamorphic  action,  and  yet  strata  inti- 
mately associated  with  them  have,  in  occasional  instances,  contained 
fossils. 

It  is  not  easy  to  fix  the  exact  upper  limit  of  this  series.  The 
fossils  are  few,  obscure,  and  seldom  met  with  in  the  lowest  fossilif- 
erous  series ;  and  the  transition  is  very  gradual  from  the  distinctly 
metamorphic  to  the  fossiliferous  rocks.  This  renders  it  impossible 
always  to  determine  accurately  the  line  of  separation. 

The  gneiss,  mica  slate  and  argillaceous  slate,  have  the  order  of 
superposition  in  which  they  are  here  named.  They  differ  only  in 
the  amount  of  metamorphic  action  to  which  they  have  been  sub- 
jected ;  and  the  gneiss  which  is  most  highly  metamorphic  has,  by 
being  the  lowest,  been  most  acted  upon,  —  the  mica  slate  less,  and 
the  argillaceous  slate  least.  In  a  particular  locality,  however,  the 


32  FOSSILIFEROUS   ROCKS. 

lowest  rock  which  was  subjected  to  these  causes  of  change,  instead 
of  having  been  of  such  a  character  as  to  produce  gneiss,  may  have 
been  a  limestone,  and  in  that  case  the  lowest  metamorphic  rock 
would  be  a  saccharine  marble.  In  another  locality  the  lowest  rock 
may  have  been  a  sandstone,  which  would  be  converted  into  quartz 
rock.  Hence  there  may  occur,  in  any  part  of  the  metamorphic 
series,  crystalline  limestone,  quartz  rock,  hornblende  slate,  chlorite 
slate,  and  talcose  slate ;  and  any  one  of  these  rocks  may  be  as 
abundant  in  any  particular  region,  as  gneiss,  mica  slate  or  argillace- 
ous slate,  is  in  another. 

The  metamorphic  rocks  occur  in  all  countries  where  there  has 
been  any  considerable  amount  of  volcanic  action,  and  their  total 
amount  is  very  great ;  but  their  stratification  is  so  confused  and 
contorted,  their  superposition  so  irregular,  and  denudations  have 
been  so  extensive,  that  no  estimate  can  be  made  of  their  thickness. 
They  are,  perhaps,  equal  to  all  the  other  stratified  rocks. 


SECTION   V.  —  THE   FOSSILIEEROUS  ROCKS. 

The  fossiliferous  rocks  are  divided  into  seven  systems,  which  are 
readily  distinguished  by  the  order  of  superposition,  lithological  char- 
acters and  organic  remains.  These  systems  are  the  Silurian,  the 
Old  Red  Sandstone,  the  Carboniferous,  the  New  Red  Sandstone,  the 
Oolitic,  the  Cretaceous,  and  the  Tertiary  systems.  There  is  also 
an  eighth  system  now  in  process  of  formation. 

It  is  the  opinion  of  some  geologists  that  there  is  another  system 
situated  between  the  metamorphic  rocks  and  the  silurian  system. 
It  has  been  called  by  Dr.  Emmons,  who  has  studied  it  with  much 
care,  the  "Taconic  System,"  the  Taconic  Mountains,  in  the 
western  part  of  Massachusetts,  being  composed  of  these  rocks.  It 
is  the  lower  part  of  what  has  been  called,  in  England  and  Wales, 
the  Cambrian  system. 

The  strata  of  this  system  have  a  nearly  vertical  position,  and 
consist  principally  of  black,  greenish  and  purple  slates,  of  great 


FOSSILI FERGUS   ROCKS. 


thickness.  Granular  quartz  rock,  however,  occurs  in  considerable 
quantity,  and  in  this  country  two  thick  and  important  beds  of 
limestone  are  found.  These  limestones  are  occasionally  white  and 
crystalline.  Generally,  however,  as  a  mass,  they  are  a  dark,  nearly 
black  rock,  with  a  network  of  lines  of  a  lighter  color.  All  the 
clouded  marbles  for  architectural  and  ornamental  purposes  are  from 
these  beds,  and  our  roofing  and  writing  slates  are  all  obtained  from 
the  argillaceous  portion  of  this  system. 

The  number  of  species  of  organic  remains  contained  in  this 
system  is  very  small,  and  these,  so  far  as  discovered,  belong  to  the 
annellida,  with  a  few  doubtful  cases  of  mollusca.  This  system  of 
rocks  is  found  coming  to  the  surface  in  a  large  part  of  New  Eng- 
land, and  the  eastern  part  of  New  York,  also  in  the  western  part 
of  England  and  Wales. 

Those  geologists  who  deny  the  existence  of  this  system  consider 
these  rocks  as  parts  of  the  silurian  system  which  have  been  most 
disturbed  by  subterranean  forces,  and  most  altered  by  proximity  to 
igneous  rocks.  The  annexed  sketch  (Fig.  7)  will  exhibit  the 
relations  here  referred  to.  Certain  portions  of  the  silurian  rocks 
are  supposed  to  have  been  thrown  into  folds  by  the  upheaval  of  the 
primary  rocks.  The  plications  nearest  to  the  intrusive  granite 
would  be  most  altered.  That  part  of  the  figure  below  the  line  a  a 

Fig.  7. 


represents  the  outcropping  edges  as  they  now  appear,  the  upper 
portion  of  the  folds  having  been  removed  by  some  abrading  cause. 
As  it  is  yet  uncertain  which  of  these  views  is  correct,  conve- 
nience will  justify  us  in  retaining  the  name  of  Cambrian  system  till 
further  investigations  shall  settle  the  question. 


34 


FOSSILIFEROUS   ROCKS. 


1.  The  Silurian  System.  —  The  following  tabular  arrangement 
exhibits  the  divisions  of  the  system  as  recognized  in  England,  in 
New  York,  in  Pennsylvania  and  Virginia,  and  in  Ohio. 


Divisions  as  recognized  by 


Divisions  as  recognized  by  the  New         Pennsylvania  Ohio. 


English  Authors. 

York  Geologists.                        and  Virginia. 

1 

Upper      Cambrian 

'  Potsdam  Sandstone.                      >   XT     .. 
>  No.  1. 

c 

§ 

Calciferous  Sandrock. 

g 

Rocks,  of  Sedgwick 

•- 

1 

(probably). 

3 

c  * 

Birdseye  Limestone.                     1  No.  2.      1 
Blue 

G 

1 

Trenton  Limestone.                     J                     Limestone 

Llandeilo  Flags.       '. 

f  ' 

Utica  Slate.                                   "j                      and  Marl. 

1 

C 

[  No.  3.      . 
-  Hudson  River  Group.                  J 

Gray  Sandstone.                           ") 
i  No.  4. 

J 

Oneida  Conglomerate.                  J 

Caradoc  Sandstone. 

a"  . 

Medina  Sandstone.                       "1 

.2 

\  No.  5. 

5 

Clinton  Group.                             J 

•5 

Niagara  Group.                            *|                   •) 

Oneida  Salt  Group. 

Water-lime  Group. 

Pentamerus  Limestone.                    No.  6. 

i 

= 

Delthyris  Shaly  Limestone. 

«  ' 

Wenlock  Rocks. 

s 

Encrinal  Limestone. 
Cliff 

vfi 

1  ' 

Upper  Pentamerus  Limestone.  J                   |  Limestone. 

1 

Oriskany  Sandstone.                     "j 

a 

Cauda-Galli  Grit.                         i  No.  7. 

Schoharie  Grit. 

Onondaga  Limestone.                      Wanting. 

. 

^  Corniferous  Limestone.                   Wanting.  -* 

-  Marcellus  Shales.                         -|                   J  Black  Slate. 

Hamilton  Group. 

Upper  and  Lower 

c 

No.  8. 

Tully  Limestone. 

c 

Ludlow  Rocks,  and  « 

^ 

.2 

"7 

Genesee  Slate. 

> 

the  Devonian  Sys- 

'f. 

a 

tem. 

Portage  Group.                             \  No  9       ]Waveriy 

I 

k  Chemung  Group.                          J                   J  Sandstone. 

FOSSILIFEROUS   ROCKS.  35 

This  name,  Silurian,  was  first  used  to  designate  the  lowest 
well-characterized  fossiliferous  rocks  in  England.  But  it  is  now 
used  to  embrace  the  whole  system  as  it  occurs  elsewhere.  It  is 
well  exhibited  in  New  York,  both  in  consequence  of  its  great 
development  there,  and  because  the  whole  system  is  only  slightly 
acted  upon  by  disturbing  forces,  so  that  the  outcropping  edge  of 
each  division  extends  over  a  large  surface. 

This  system  is  of  great  thickness,  amounting,  in  places  where 
it  is  well  developed,  to  twenty  thousand  feet. 

The  Champlain  division  commences  with  a  quartzose  sandstone, 
passing  gradually  into  limestone,  which  is  succeeded  by  a  very 
thick  argillaceous  deposit,  the  Utica  slate  and  Hudson  River  group. 
The  Ontario  division  in  the  lower  part  is  a  mass  of  sandstone. 
Above  this  is  the  Clinton  group,  consisting  of  shales  and  sand- 
stones. The  most  important  part  of  this  group,  in  an  economical 
point  of  view,  is  a  fossiliferous,  argillaceous  iron  ore,  coextensive 
with  the  group  in  this  country,  and  is  worked  to  supply  a  large 
number  of  furnaces.  The  last  of  the  division  is  the  Niagara  group, 
which  commences  with  a  mass  of  shale,  and  becoming  at  length 
calcareous,  it  terminates  in  a  firm  compact  limestone.  This  lime- 
stone has  withstood  the  action  of  denuding  causes  better  than  the 
shales  either  above  or  below  it.  It  therefore  presents  a  bold 
escarpment  at  its  outcrop,  and  occasions  waterfalls  wherever 
streams  of  water  cross  it.  The  falls  of  Niagara  are  formed  by  this 
rock.  The  Niagara  limestone,  in  its  extension  westward,  becomes 
the  lead-bearing  rock  of  Missouri,  Iowa  and  Wisconsin.  The 
Helderberg  division  is  a  succession  of  highly  fossiliferous  limestones, 
with  the  intervention  of  only  occasional  beds  of  grits  and  shales. 
One  member  of  the  series  is  the  Onondaga  Salt  group.  The  water 
obtained  from  this  group  in  New  York  annually  furnishes  immense 
quantities  of  salt.  The  Erie  division  consists  of  a  thick  mass  of 
shales  and  sandstones. 

The  fossils  of  this  system  are  very  numerous,  but  consist  mostly 
of  the  lower  forms  of  animal  life.  Corals  (Figs.  8  and  9)  are 


36 


FOSSILIFEROUS   ROCKS. 


Pig.  8. 


abundant,  and  constitute   in  some  places  a  large  proportion  of 
the   limestones.     The  Crinoidea,  or  lily-shaped  animals,  consist 

of  a  jointed  stem  permanently  at- 
tached, and  bearing  at  the  free 
extremity  of  the  stem  an  expand- 
ed portion,  which  is  the  pelvis,  or 
digestive  cavity.  The  mouth  is 
surrounded  with  a  series  of  leaf- 
like  tentacula,  which  serve  the  pur- 
pose of  seizing  and  holding  food. 
Fig.  10  represents  the  pelvis  of 
one  of  the  silurian  fossils.  The 
general  character  of  the  animal  is 
better  represented  by  Fig.  30. 
The  most  abundant  fossils  of  this 
period  are  the  lowest  orders  of 
bivalve  mollusca  (Fig.  11).  The 
Cephalopoda  are  characterized  by 
having  the  organs  of  locomotion  attached  to  the  head.  The  shell 
of  several  species  is  peculiar  in  being  divided  into  distinct  cells,  or 

Fig.  9. 


Fig.  10. 


chambers  (Fig.  12,  b  d),  perforated  by  a  tube  (siphuncle  a).   These 
fossil  shells  are  sometimes  straight,  as  the  Orthoceras  (Fig.  13),  or 


FOSSILIFEROUS   ROCKS. 


37 


curved,  as  shown  in  the  several  forms  of  Fig.  14.     The  Trilobite 

Fig.  11. 


was  an  articulated,  crustaceous  animal,  having  two  lines  along  the 

Fig.  15. 


Fte.  12. 


back  dividing  it  into  three  lobes,  from  which  circumstance  its  name 
4 


38 


FOSSILIFEEOUS   ROCKS. 


Fig.  13. 


Kg.  14. 


is  derived.  It  is  found  in  great 
numbers  in  the  Silurian  rocks 
(Fig.  15).  In  a  few  instances 
remains  of  fishes  have  been 
found,  but  they  by  no  means 
characterize  the  system. 

The  geographical  range  of 
this  system  is  probably  great- 
er than  that  of  any  system  of 
rocks  above  it.  It  is  found 
occupying  a  large  part  of  the 
territory  west  of  the  Allegha- 
ny  Mountains,  from  Canada, 
through  New  York,  and  the 
other  states,  to  Alabama ;  and 
extending  westward  to  and 
beyond  the  Mississippi  river. 
It  occupies  a  large  district  in 
the  west  of  England,  and  is 
found  in  great  force  in  the 
north  and  east  of  Europe. 
2.  The  Old  Red  Sandstone.  —  This  formation  consists  almost 
entirely  of  a  sandstone  of  a  red  color.  It  admits  of  division  into 
three  parts,  though  the  characters  vary  in  different  places.  The 
lowest  is  a  thin-bedded  argillaceous  sandstone,  consisting  of  finely 
levigated  material,  and  easily  splitting  into  thin  sheets.  From  this 
circumstance  it  has  received  the  name  of  tilestone.  The  middle 
portion  is  composed  of  nodules  or  concretions  of  limestone  im- 
bedded in  a  paste  of  red  sand  and  shale.  This  has  been  called  by 
English  geologists,  cornstone^  and  though  very  partially  developed 
in  some  regions  where  the  system  is  found,  it  is  yet  a  very  persistent 
member.  The  highest  member  of  this  formation  is  a  mass  of  red 
sandstone,  often  passing  into  a  coarse  conglomerate.  In  England 
the  thickness  of  the  Old  Red  Sandstone  is  not  less  than  ten  thou- 
sand feet.  In  this  country  it  is  scarcely  three  thousand  feet. 


FOSSILIFEROUS   ROCKS. 


39 


The  fossils  of  this  system  are  a  few  shells,  a  small  number  of  veg- 
etable species,  and  in  particular  localities  the  remains  of  fishes  in 
great  abundance.  The  system  is  char- 
acterized  principally  by  fossils  of  this 
last  kind.  The  fishes  of  this  system 
have  a  cartilaginous  skeleton,  but  are 
covered  with  plates  of  bone,  which 
were  faced  externally  with  enamel. 
The  jaws,  which  consisted  of  solid 
bone,  were  not  covered  with  integu- 
ment. The  exterior  bony  covering 
seems  to  have  been  the  true  skeleton, 
as  is,  in  part,  the  case  with  the  tor- 
toise. In  some  of  the  fishes  of  this 
period  there  is  a  wing-like  expansion 
on  each  side  of  the  neck,  which  has 
given  them  the  name  of  Pterych- 
this  (Fig.  17).  In  others,  as  the  Ce- 
pkalaspis,  the  plate  of  bone  on  the 
back  is  so  large  as  to  cover  nearly 
the  whole  body,  and  make  it  resemble 
a  trilobite  (Fig.  IB). 

This  system  has  an  extensive  geographical  range.  In  England, 
it  occupies  a  band  of  several  miles  in  width,  extending  from  the 
Welsh  border  northward  through  Scotland  to  the  Orkney  Islands. 
In  this  country,  it  forms  the  Catskill  Mountains,  in  New  York, 
and  extends  south  and  west  so  as  to  underlie  the  coal-fields  of 
Pennsylvania  and  Virginia. 

3.  The  Carboniferous  System.  —  This  system  consists  of  three 
parts,  distinguished  by  lithological  and  fossil  characters. 

The  carboniferous  limestone  is  a  dark-colored,  compact  lime- 
stone, forming  the  base  of  the  system,  and  reposing  on  the  old  red 
sandstone.  Its  thickness  is  from  six  hundred  to  one  thousand 
feet,  often  with  scarcely  any  intermixture  of  other  rock  ;  but  it 
sometimes  loses  its  character  of  a  limestone,  and  becomes  a  sand- 


40 


FOSSILIFEROUS   BOCKS. 


stone,  or  conglomerate.     It  generally  contains  the  ores  of  lead  in 
considerable  quantity,  and  from  this  circumstance  has  been  called 

Fig.  17. 


metalliferous  limestone.  In  England  it  is  the  principal  reposi- 
tory of  these  ores.  In  the  Western  States  it  is  the  upper  portion 
of  the  lead-bearing  strata. 

The  fossils  are  marine,  and  very  numerous.  Corals  and  cri- 
noidea  are  very  abundant.  The  crinoidea,  in  some  localities,  form  so 
large  a  part  of  the  rock  as  to  have  given  to  it  the  name  of  encrinal 
limestone.  The  orthoceras  and  trilobite  are  found,  but  become 
extinct  with  this  formation.  Several  species  of  bivalves,  such  as 
Delthyris  and  Leptaena,  are  also  common. 

Next  above  the  limestone  lies  the  sandstone,  sometimes  called 
millstone  grit.  It  is  generally  drab-colored,  but  occasionally  red. 
Its  thickness  is  often  equal  to  that  of  the  limestone.  Sometimes 


FOSSILIFEROUS   BOCKS. 


41 


it  is  fine-grained  and  compact ;  but  generally  it  is  coarse-grained, 
and  often  passes  into  a  conglomerate.  It  contains  but  few  fossils, 
and  those  of  vegetable  origin. 

The  highest  part  of  the  system  is  the  coal  measures.  They 
consist  of  beds  of  sandstone,  limestone,  shale,  clay,  ironstone  and 
coal,  occurring  without  much  uniformity  in  their  order  of  super- 
position. The  coal  measures  have  a  thickness  of  about  three  thou- 
sand feet.  The  sandstones  and  limestones  are  not  distinguishable 
from  the  sandstones  and  limestones  in  the  lower  part  of  the  sys- 
tem. The  ironstone  either  occurs  in  concretionary  nodules,  often 
formed  around  some  organic  nucleus,  or  it  is  an  argillaceous  ore, 
having  a  slaty  structure.  In  either  case,  it  consists  of  subordinate 
beds  in  the  shale.  The  coal  consists  of  several  beds  distributed 
through  the  measures.  The  beds  vary  in  thickness  from  a  few 
lines  or  inches  fo  several  feet.  -Jn  a  few  eases  beds  have  been 
found  measuring  fifty  or  sixty  feet  in  thickness.  The  workable 
beds  are  ordinarily  from  three  to  six  feet  thick. 

The  carboniferous  formation  is  very  much  disturbed  by  dikes, 
Flg  18.  fault*    (Fig.    18; 

see  also  Fig.  50), 
and  other  disloca- 
tions. The  amount 
of  change  of  posi- 
tion in  the  strata,  by 
faults,  is  very  va- 
rious ;  frequently 
but  a  few  feet.  In 
one  case  in  England  there  is  a  fault  of  nearly  a  thousand  feet. 
There  is  a  case  of  dislocation  in  Belgium  where  the  strata  are  bent 
into  the  form  of  the  letter  Z,  so  that  a  perpendicular  shaft  would 
cut  through  the  same  bed  of  coal  several  times. 

The  characters  and  order  of  superposition  which  have  now  been 

given  may  be  regarded  as  the  general  type  of  the  carboniferous 

formation.     There  are,  however,  several  important  modifications. 

1.  Beds  of  coal  sometimes  alternate  with  beds  of  millstone  grit. 

4* 


42  FOSSILIFEROUS   ROCKS. 

Thus,  in  Scotland  and  in  the  north  of  England,  this  intermediate 
member  of  the  system  disappears,  or,  rather,  is  incorporated  with 
the  coal  measures.  The  same  is  true,  to  considerable  extent,  in 
this  country.  2.  Sometimes  the  carboniferous  limestone  also  dis- 
appears as  a  distinct  member  of  the  system,  partly  by  becoming 
arenaceous,  and  partly  by  the  intercalation  of  beds  of  coal.  In 
this  last  case,  the  whole  formation  from  the  old  to  the  new  red 
sandstone  becomes  a  series  of  coal  measures.  In  this  country  the 
carboniferous  limestone  is  found  very  generally  to  underlie  the 
coal  strata.  3.  The  fractures  and  faults,  which  were  formerly  sup- 
posed to  be  characteristic  of  the  coal  formation,  are  seldom  found 
in  the  great  coal-fields  of  this  country,  except  in  those  of  the 
anthracite  coal  of  Pennsylvania ;  and  even  there  they  are  much 
less  common  than  in  the  coal-fields  of  Europe. 

There  are  three  principal  varieties  of  coal,  distinguished  by  the 
different  proportions  of  bitumen  which  they  contain.  The  common 
bituminous  coal  kindles  readily,  emits  much  smoke,  and  throws 
out  so  much  liquid  bitumen  that  the  whole  soon  cakes  into  a  solid 
mass.  It  contains  about  forty  per  cent,  of  bitumen.  The  second 
kind,  or  cannel  coal,  contains  twenty  per  cent.,  and  inflames  easily, 
but  does  not  agglutinate.  The  stone-coal,  or  anthracite,  contains 
scarcely  any  bitumen,  ignites  with  difficulty,  emits  but  little  smoke, 
and  produces  a  very  intense  heat.  The  bituminous  varieties  are 
always  found  in  the  least  disturbed  portions  of  the  coal  districts  ; 
and  the  anthracite  is  found  in  the  more  broken  and  convulsed  por- 
tions, where  we  may  suppose  that  the  subterranean  heat  has  been 
sufficient  to  drive  off  the  volatile  bituminous  part,  and  reduce  it 
to  the  anthracite  form.  Hence  the  eastern  Pennsylvania  coal- 
fields, which  lie  near  the  principal  axes  of  elevation  of  the  Appa- 
lachian Mountains,  furnish  only  anthracite ;  while  the  same  coal- 
seams,  in  their  extension  to  the  western  part  of  the  state,  are 
bituminous. 

Where  coal  is  quarried  in  large  quantity,  a  shaft  is  sunk  through 
the  overlying  strata  to  the  coal-beds,  and  the  coal  is  raised  to  'the 
surface  by  steam  power.  After  the  coal  has  been  quarried  to  some 


FOSSILIFEROUS   ROCKS.  43 

distance  from  the  shaft,  pillars  of  unquarried  coal  are  left  to  sup- 
port the  overlying  strata.  Fatal  accidents  have  sometimes  occurred 
by  the  giving  away  of  these  supports.  Over  a  large  part  of  the 
coal-fields  of  the  United  States  it  has  not  yet  become  necessary  to 
sink  shafts.  The  quarrying  is  commenced  at  the  outcrop  of  the 
coal-bed ;  and,  till  the  cover  becomes  of  considerable  thickness,  it 
has  been  found  economical  to  "  strip  "  off  the  overlying  rock,  rather 
than  to  work  a  subterranean  gallery. 

Brine-springs  are  often  found  in  the  coal  measures  of  sufficient 
strength  to  be  used  in  the  manufacture  of  salt.  This  is  now  done 
to  considerable  extent  in  Ohio.  In  the  valley  of  the  Kenhawa 
river,  Kentucky,  the  rocks  of  which  belong  to  the  carboniferous 
system,  the  brine  is  nearly  saturated  with  salt ;  and  in  some  of  the 
borings  they  have  even  discovered  beds  of  rock-salt  of  great  thick- 
ness and  purity. 

There  is  no  other  part  of  the  geological  series  so  obviously  con- 
nected with  national  prosperity  as  the  coal  formation.  While  a 
country  is  new,  the  forests  furnish  an  abundant  supply  of  fuel ;  but 
in  the  course  of  a  few  years  these  are  consumed.  This  country  will 
soon  be  principally  dependent  upon  its  coal-mines  for  fuel,  even  for 
domestic  purposes ;  and,  in  carrying  on  the  great  branches  of 
national  industry,  such  as  the  smelting  and  working  of  iron,  and 
in  the  formation  of  steam  for  the  purposes  of  manufacture  and 
transportation,  we  are  already  mainly  dependent  upon  mineral 
coal.  A  nation  which  does  not  possess  an  abundant  supply  of  this 
mineral,  or  which  does  not  use  it,  cannot  long  maintain  a  high 
degree  of  national  prosperity. 

In  these  inexhaustible  masses  of  coal,  accumulated  ages  before 
the  existence  of  the  human  race,  is  a  most  obvious  prospective 
arrangement  for  securing  our  happiness  and  improvement.  And 
this  arrangement  embraces  not  only  the  accumulation  of  a  com- 
bustible material  in  such  abundance,  but  also  its  juxtaposition  with 
an  equally  inexhaustible  accumulation  of  iron  ore,  and  the  limestone 
which  is  necessary  as  a  flux  in  the  reduction  of  the  ore.  So  bulky 
and  heavy  materials  as  coal  and  iron  ore  could  neither  of  them 


44 


FOSSILIFEROUS   ROCKS. 


have  been  transported  to  any  considerable  distance  for  the  manu- 
facture of  iron  ;  and  without  the  manufacture  of  iron  on  a  large 
scale,  the  present  operations  in  manufactures  and  transportation 
could  never  have  been  entered  upon.  A  large  proportion  of  the 
iron  furnaces  in  this  country,  and  nearly  all  of  them  in  Great 
Britain,  employ  mineral  coal  for  fuel,  and  obtain  their  ore  from 
the  beds  contained  in  the  coal  measures. 

The  fossils  of  the  coal  measures  are  almost  entirely  of  vegetable 
origin,  and  are  very  abundant.  They  are  seldom  found  in  the 
coal-beds,  but  in  the  strata  of 
shale  immediately  above  or 
below  the  solid  coal. 


Fig.  19. 


The  Stigmaria  (Fig.  19)  is  found  most  abundantly,  and  in  a 
large  proportion  of  cases  to  the  exclusion  of  every  other  form,  in 
the  lower  shales.  It  consisted  of  a  large  dome-shaped  mass,  often 


FOSSILIFE110US   ROCKS. 


45 


three  or  four  feet  in  diameter,  with  trailing  branches,  or  roots, 
spreading  off  horizontally  to  a  distance  of  twenty  feet.  In  a 
few  instances  tree  ferns  have  been  found,  petrified  in  a  horizontal 
position,  and  being  apparently  a  mere  continuation  of  the  stigmaria. 
Hence  the  stigmaria  has  been  supposed  to  be  the  base  of  the  tall 
tree  ferns,  the  leaves  of  which  so  abound  in  the  upper  shales.  If 
this  is  not  the  case,  there  are  no  forms  of  the  existing  flora  of  the 
earth  analogous  to  the  stigmaria.  It  is  always  found  in  connection 
with  the  coal-beds  of  the  carboniferous  formation,  and  never  with 
the  coal-beds  which  sometimes  occur  in  the  later  formations. 

The  tree  ferns  (Fig.  20)  attained  a  height  of  fifty  or  sixty  feet, 
and  a  diameter  of  four  feet.  They  have  received  the  name  of 
Sigittaria  in  consequence  of  the  seal-like  impressions  (Fig.  21) 

Fig.  20. 

Pig.  21. 


with  which  the  surface  is  covered,  and  which  are  the  scars  left 
where  the  fronds  have  fallen  off.  These  fronds  (fern  leaves)  are 
the  most  abundant  fossil  of  the  series.  They  are  distinguished  by 
some  peculiarity  in  form,  as  the  Sphenopteris  (wedge-shaped  fern 
leaf),  Pachypteris  (thick  fern  leaf),  &c.  (Figs.  22  and  23.) 


46 


FOSSILIFEROU3   ROCKS. 


Fig.  22. 


Fig.  23. 


Fig.  24. 


There  was  another  kind 
of  Sigillaria  (Fig.  24),  in 
which  the  surface  was  fluted, 
and  the  markings  are  super- 
ficial, and  occur  on  the 
ridges.  It  reached  as  great 
a  size  as  the  tree  ferns,  but 
to  what  general  class  of 
plants  it  belonged  is  still 
doubtful. 

The  Lepidodendron  (scale-covered  tree)  (Fig.  25)  is  the  fossil 
which  most  nearly  resembled  in  general  appearance  our  present 


FOSSILIFEROUS   ROCKS.  47 

forest  trees.  Specimens  are  found  four  feet  in  diameter  and 
seventy  feet  in  height.  In  botanical  characters  it  resembled,  in 
some  respects,  the  trailing  club-mosses,  while  in  others  it  was  very 
similar  to  the  Norfolk  Island  pine. 

The  Catamite  (Fig.  26)  was  a  plant  resembling,  in  its  jointed 


Pig.  26. 


and  striated  surface,  the  equisetum  (rush),  but  was  sometimes 
twelve  inches  in  diameter. 

The  carboniferous  formation  exists  more  or  less  abundantly  in 
all  the  great  divisions  of  the  earth.  It  occurs  in  nearly  all  of  the 
countries  of  Europe.  The  largest  deposits  known  are,  however,  in 
the  United  States ;  especially  in  the  States  of  Pennsylvania  and 
Virginia,  and  in  Ohio. 

4.  The  New  Red  Sandstone.  —  The  lower  division  of  this  form- 
ation, called  the  Permian  system,  consists  of  a  thick  mass  of  sand- 
stones, generally  of  a  red  color,  with  occasional  alternations  of 
argillaceous  rock,  succeeded  by  a  series  of  magnesian  limestones. 
The  upper  division,  or  Triassic  system,  is  composed  of  a  red  con- 
glomerate, a  limestone  which  has  received  the  name  of  Muschel- 
kalk  (shelly  limestone),  and  a  series  of  variegated  marls  and  sand- 
stones. 

The  ores  of  copper  are  found,  to  considerable  extent,  in  this 
formation.  The  rich  copper  mines  of  Germany  are  in  the  mag- 
nesian limestone,  or,  as  it  is  there  called,  Zechstein  (minestone). 
The  Lake  Superior  copper  mines  occur  in  a  red  sandstone  forma- 
tion, which  will  probably  be  found  to  belong  to  this  system. 

The  salt-beds,  salt  springs,  and  beds  of  gypsum,  are  so,  gen- 
erally found  in  this  rock  in  England,  that  it  has  been  called  by 
the  English  geologists  the  "  saliferous  system."  It  is,  however, 
found  that  in  other  countries  these  minerals  occur  in  equal  abun- 
dance in  formations  of  an  earlier  and  later  date. 


48 


FOSSILIFEROUS   ROCKS. 


The  fossils  of  this  system  are  not  abundant.     In  the  Permian 
portion,  impressions  of  fishes  are  found,  always  with  the  peculiarity 
that  the  tail  is  heterocercal  (Fig.  27) ;  that  is,  with  the  spine  con- 
Fig.  27. 


tinned  into  the  upper  lobe.  The  same  peculiarity  prevails  in  the 
carboniferous  and  all  the  earlier  formations.  Fishes  with  the  tail 
komocercal  begin  to  appear  in  the  Triassic  portion  of  this  system, 
and  are  found  in  all  the  subsequent  formations.,  The  remains  of 
saurians  also  occur  in  this  formation. 

The  red  sandstones  seem  to  have  been  better  adapted  to  retain 
the  forms  which  were  impressed  upon  them  than  to  preserve  the 
organic  remains  which  were  deposited  in  them.  Hence,  while 

Fig.  28. 


they  contain  but  few  fossils,  the  strata  are  often  covered  with 
ripple  marks,  with  sun  cracks,  occasioned  by  contraction  while  dry- 
ing, or  with  depressions  produced  by  rain-drops,  and  the  pits  are 


FOSSILIFEROUS   ROCKS.  49 

sometimes  so  perfect  as  to  show  the  direction  of  the  wind  when 
the  drops  fell.  (Fig.  28.)  The  tracks  of  animals  are  also  well 
preserved.  Some  of  them  were  produced  by  reptiles  (Fig.  29, 
c),  and  some  probably  by  marsupial  animals,  but  most  of  them 
by  birds  (a,  b).  President  Hitchcock  has  distinguished  the  tracks 
Fig.  29. 


m 


of  more  than  thirty  species  in  the  sandstones  of  the  Connecticut 
valley.  Birds,  reptiles  and  marsupial  animals,  seem  to  have  been 
first  introduced  during  this  period. 

The  new  red  sandstone  is  well  developed  in  all  its  members  on 
the  continent  of  Europe.  In  England,  all  the  members  are  pres- 
ent, except  the  Muschelkalk.  The  Triassic  portion  of  it  occurs  in 
North  America.  It  is  found  in  detached  portions,  probably  as 
parts  of  a  continuous  formation,  in  Nova  Scotia,  the  eastern  part 
of  Maine,  the  Connecticut  valley,  and  from  New  Jersey  southward 
through  Pennsylvania,  Maryland,  &c.,  to  South  Carolina. 

V.  The  Oolitic  System.  —  The  lower  portion  of  this  system  is 
the  Lias,  and  consists  of  a  series  of  fissile,  argillaceous  limestone, 
marl,  and  clays.  The  Oolite  forms  the  intermediate  member  of  the 
system,  and  consists  of  alternations  of  clay,  arenaceous  rock  and 
limestone.  Some  of  the  limestones  have  an  oolitic  structure,  and 
the  whole  system  takes  its  name  from  this  circumstance,  though 
this  structure  is  not  found  in  all  parts  of  it,  and  is  often  found  in 
other  formations.  The  central  part  of  the  oolite,  the  coral  rag,  is 
principally  a  mass  of  corals  and  comminuted  shells.  The  Wealden, 
the  highest  member  of  the  oolitic  system,  is  an  estuary  deposit, 
consisting  of  calcareous  beds,  followed  by  sandstone,  and  termi- 
nated by  the  Wealden  clay. 

This  system  is  throughout  highly  calcareous,  and  furnishes, 
wherever  it  is  developed,  valuable  materials  for  architectural  and 
ornamental  purposes. 

This  system  is  distinguished  for  the  great  amount  and  variety 
5 


50 


FOSSILI FERGUS   ROCKS. 


of  its  organic  remains.     The  vegetable  productions  were  inter- 
mediate between  those  of  the  coal  period  and  those  of  the  present 

time.  The  upper  oolite,  in  the 
south  of  England,  contains  the 
stumps  of  trees  and  other  plants, 
rooted  in  a  black  carbonaceous 
layer,  evidently  the  soil  from  which 
they  grew.  These  stumps  and  pros- 
trate trunks  are  the  remains  of 
coniferous  trees  of  large  growth. 
(Fig.  30.) 

Fig.  30. 


Corals  occur  in  great  abundance ; 
also  enerinites  (Fig.  31),  mollusks  (Fig. 
32),  and  cephalopoda. 

But  this  system  is  specially  charac- 
Fig.  32. 


terized  by  the  remains  of  saurian 
reptiles.  The  Ichthyosaurus  (Fig. 
33,  a)  was  a  marine  animal,  having  the  general  form  of  a  fish, 
while  its  head,  and  especially  its  teeth,  resemble  those  of  the  croc- 


FOSSILIFEROUS   ROCKS.  51 

odile.  It  was  an  air-breathing  animal  like  the  cetacea,  and  was 
furnished  with  similar  paddles.  It  was  carnivorous,  and  was 
undoubtedly  the  largest  and  most  formidable  animal  existing  in 
the  earlier  part  of  the  oolitic  period.  Its  length  could  not  have 
been  less  than  thirty  or  forty  feet. 

The  Plesiosaurus  (Fig.  33,  b)  was  also  a  marine  animal,  and  in 
ninny  respects  similar  to  the  Ichthyosaurus ;  but  its  general  form 

Fig.  33. 


was  more  slender,  its  head  was  small,  and  its  neck  was  of  great 
length,  the  cervical  vertebrae  exceeding  in  number  those  of  the 
swan. 

The  Pterodactyls  (Fig.  34)  was 
a  small  saurian,  of  the  size,  proba- 
bly, of  our  largest  eagle.  The 
finger-bones,  which  in  the  other 
saurians  form  the  paddles,  are  in 
the  Pterodactylg  very  much  length- 
ened, so  as  to  support  a  membran- 
ous expansion,  like  that  of  the  bat. 
These  wings  were  of  sufficient  size  to  enable  it  to  sustain  itself 
in  the  air,  and  to  make  a  rapid  and  easy  flight. 

The  Iguanodon  is  a  Wealden  fossil,  remarkable  for  it1?  great 
magnitude.  It  is  estimated  that  its  length  was  seventy  feet.  It 
was  a  lizard,  adapted  for  motion  on  land,  and  was  herbivorous. 

This  "formation  is  well  developed  in  England,  and,  with  the 
exception  of  the  Wealden,  on  the  continent  of  Europe.  It  has  been 
supposed  that  no  part  of  the  oolitic  series  was  to  be  found  in  this 
country;  but  there  is  a  highly  arenaceous  rock  occupying  the 
valley  of  the  James  river,  in  the  vicinity  of  Richmond,  Virginia, 
of  considerable  extent,  and  a  thousand  feet  in  thickness,  containing 


FOSSILIFEROUS   HOCKS. 


a  bed  of  coal  of  forty  feet  in  thickness,  which,  from  its  fossils,  must 
be  referred  to  the  oolitic  series. 

6.  The  Cretaceous  Formation.  —  The  lower  part  of  this  forma- 
tion consists  of  greensand,  interstratified  with  beds  of  clay.  The 
intermediate  portion  is  a  mixture  of  argillaceous  green-sand  and 
impure  chalk.  The  upper  part  is  composed  of  chalk,  which  is  a 
friable,  nearly  pure  carbonate  of  lime.  The  strata  of  chalk  are 
separated,  at  intervals  of  from  three  to  six  feet,  by  layers  of  flint, 
either  in  the  form  of  nodules  or  of  continuous  strata. 

These  characters,  by  which  the  cretaceous  system  is  known  in 
England,  are  but  partially  recognized  elsewhere.  Thus,  in  the 
Alps,  the  "  Neocomian  System,"  consisting  of  crystalline  limestones, 
is  the  equivalent  of  the  English  greensand ;  while  the  greensand 
of  this  country  is  the  equivalent  of  the  white  chalk  of  England. 

The  fossils  of  the  cretaceous  formation  are  very  different  from 
those  of  the  oolite,  and  are  such  as  to  show  that  it  was  deposited 
in  deep  seas.  Microscopic  shells  are  often  so  abundant  as  to  con- 
stitute a  large  proportion  of  the  mass.  Zoophytes  are  very 

Fig.  35. 


numerous,  such  as  sponges,  corals,  star-fishes  (Fig.  35,  d  e),  and 
a  few  crinoidea  (b).  Mollusks  were  also  abundant,  and  cephalo- 
Rg.  36.  poda,  consisting  of  cham- 

ber-shells and  belemnites 
(Fig.  36).  The  belemnite 
probably  resembled  the  ex- 
isting cuttle-fish;  but  the 
remains  consist,  in  most 
cases,  of  a  partially  hollow  calcareous  substance  (/>),  which  was  con- 
tained within  the  animal,  and  formed  its  skeleton. 

The  chalk  and  greensand  are  largely  developed  in  England  ; 


FOSSILIFEROUS   ROCKS.  53 

and  the  same  formation,  with  different  lithological  characters,  is 
found  in  great  force  flanking  the  principal  mountain  ranges  of 
southern  Europe,  and  extending  into  Asia.  In  this  country  the 
system  commences  with  the  greensand  and  friable  limestones  of 
New  Jersey,  and  following  the  Alleghany  range  to  its  southern 
termination,  it  bends  around  into  a  north-western  direction,  and  is 
continued  into  Missouri. 

7.  The  Tertiary  System.  —  The  tertiary  strata  embrace  the 
formations  from  the  cretaceous  to  the  human  era.  They  consist 
of  clay,  sand,  sandstone,  marl  and  limestone,  and  are  distinguished 
from  the  lower  rocks  by  being  less  consolidated ;  though  the  lime- 
stones are  in  some  instances  solidified,  and  resemble  the  strata  of 
earlier  origin.  The  tertiary  strata  are  generally  of  less  thickness 
than  the  older  formations,  and  less  continuous,  being  local  deposits 
formed  in  lakes  and  estuaries.  In  a  few  instances  they  have 
been  thrown  into  inclined  positions,  though  in  most  cases  they 
have  been  but  slightly  disturbed,  and  raised  but  a  few  hundred 
feet  above  the  present  level  of  the  sea. 

The  late  tertiary  strata  seldom  overlap  the  older,  so  as  to  indi- 
cate their  relative  ages  by  superposition.  They  have  therefore  been 
separated  into  groups  according  to  the  proportions  of  living  and 
extinct  species  of  shells  which  they  are  found  to  contain.  The 
oldest  tertiary  or  Eocene  formation*  contains  only  four  per  cent, 
of  living  species,  the  Miocene  contains  seventeen  per  cent.,  the 
Pleiocene  forty  per  cent.,  and  the  Pleistocene  ninety  per  cent. 

During  the  pleistocene  period,  peculiar  conditions  existed,  by 
which  a  great  amount  of  loose  material,  known  by  the  name  of 
drift,  was  spread  over  the  northern  portions  of  both  hemispheres. 
In  America  it  is  found  from  Nova  Scotia  nearly  to  the  Rocky 
Mountains,  and  extending  as  far  south  as  Pennsylvania  and  the 
Ohio  river.  In  Europe,  it  is  found  from  the  Atlantic  to  the  Ural 

*  Eos,  dawn,  and  kainos,  recent.     The  formation  which  commenced  at  the 
dawn  of  the  recent  period,  containing  but  a  small  number  of  living  species. 
Miocene  (meion,  less),  less  recent  than  the  Pleiocene  (pleion,  more). 
Pleistocene  (pUistos,  most),  most  recent. 

5* 


54  FOSSIL1FEROUS    ROCKS. 

Mountains,  and  reaching  south  into  Germany  and  Poland.  It  is 
also  found  in  the  colder  portions  of  South  America,  and  in  the 
vicinity  of  several  mountains,  as  the  Alps. 

It  consists  of  irregular  accumulations  of  earthy  substances 
of  different  degrees  of  fineness,  but  characterized  by  containing 
masses  of  rock  of  considerable  size,  often  of  many  tons  weight, 
called  boulders.  Rocks  having  the  same  lithological  characters 
exist  in  situ  north  of  where  the  boulders  and  other  drift  are  now 
found,  though  at  a  distance  often  of  one  or  two  hundred  miles. 
There  can  be  no  doubt  but  that  the  drift  has  been  transported  from 
these  northern  localities;  and  the  polished,  striated  and  grooved 
condition  of  the  rocky  surface,  wherever  the  drift  is  distributed, 
has  obviously  been  produced  by  the  passage  of  the  drift  materials 
over  it. 

Towards  the  close  of  this  period,  while  the  land  was  a  few  hun- 
dred feet  below  its  present'  level,  there  were  deposited  in  the  val- 
leys of  the  drift  region  beds  of  blue  and  gray  clay,  materials 
which  are  used  in  making  bricks  and  coarse  pottery ;  also  beds  of 
sand,  sometimes  evenly  spread  out,  but  often  thrown  into  irregular 
mounds  and  ridges. 

In  regions  which  are  not  covered  with  drift,  —  as  the  south  of 
Europe  and  the  United  States,  —  the  pleistocene  deposits  are 
succeeded,  without  apparent  change  of  conditions,  by  those  which 
are  now  taking  place. 

The  formations  of  the  tertiary  period  are  distinguished  from 
those  of  the  cretaceous  period  by  the  absence  «of  deep-sea  fossils, 
and  from  the  oolite  by  the  absence  of  its  characteristic  saurians. 
The  mollusks  are  also  very  different,  such  genera  as  the  cere- 
thium  (Fig.  37),  murex  (Fig.  38),  and  conus  (Fig.  39),  which 
abound  in  the  present  seas,  first  appearing  in  the  tertiary  period. 
The  nummulite  (Fig.  40),  a  peculiar  form  of  chambered  shell,  is 
so  abundant  as  to  constitute  in  some  places  almost  the  entire  rock. 

The  period  is  however  characterized  by  the  existence  of  a  large 
number  of  pachydermatous  animals,  of  which  the  tapir,  hog,  horse 
and  elephant,  are  examples  of  living  species. 


FOSSILIFEROU3   ROCKS. 


55 


The  Paleotherium  (Fig.  41)  resembled,  in  most  respects,  the 
tapir.     It  was  furnished  with  a  short  proboscis,  and  the  foot  was 


Fig.  37. 


Fig.  38. 


Fig.  39. 


Fig.  40. 


divided  into  three  toes.  The  length  of  the  largest  species  was 
about  that  of  the  horse ;  but  its  body  was  larger,  and  it  was  of  less 
height. 

Fig.  41. 

Fig.  42. 


The  Anoplotherium  (Fig.  42)  was  a  more  slender  animal,  and 
resembled  in  size  and  general  form  the  gazelle. 

The  Megatherium,  an  animal  of  the  late  tertiary  epoch,  was 
larger  than  the  existing  species  of  elephant,  and  in  its  general 
structure  and  habits  resembled  the  sloth. 

The  Mastodon  (Fig.  43)  lived  during  the  latest  portion  of  the 
tertiary  epoch.  Its  remains  are  found  most  abundantly  where  the 
animal  seems  to  have  perished  by  sinking  into  the  soft  marshy 


56  FOSSILIFEKOUS   ROCKS. 

ground  near  the  brackish  springs  of  New  York  and  Kentucky. 
But  they  are  found  also  in   Europe  and   Asia.     It  was   larger 

Kg.  43. 


than  any  existing  land  animal,  and  was  nearly  allied  in  struc- 
ture and  habits  to  the  elephant. 

The  Mammoth  was  a  species  of  elephant,  now  extinct,  of  which 
remains  are  found  with  those  of  the  mastodon,  but  in  the  greatest 
abundance  in  Europe  and  Asia.  A  large  number  of  skeletons, 
many  of  them  imperfect,  have  been  discovered  in  the  low  grounds 
in  the  south-east  of  England.  It  was  this  animal  which  was 
found  encased  in  ice  and  sand  in  Siberia,  in  1804. 

Contemporaneously  with  the  existence  of  these  huge  animals,  a 
near  approach  was  made  to  the  present  fauna  of  the  earth,  by  the 
introduction  of  ruminant  animals  resembling  the  ox  and  deer,  and 
especially  by  the  existence  of  the  class  of  animals  which  in  ana- 
tomical characters  stands  next  to  man,  the  apes  and  monkeys. 

The  tertiary  system,  though  not  generally  so  continuous  over 
extended  areas  as  the  older  formations,  yet  constitutes  the  surface 
of  a  very  large  part  of  Europe.  (See  Fig.  59.)  In  the  United 
States  the  earlier  portion  is  found  along  the  seaboard,  from  New 
Jersey  to  Louisiana,  and  extending  back  towards  the  mountains  to 
a  distance  varying  from  ten  to  one  hundred  miles.  The  later 


FOSSILS.  57 

deposits  are  found  in  detached  portions  throughout  the  Eastern  and 
Middle  States.  It  covers  a  large  surface  in  South  America,  and 
is  found  in  India. 

8.  The  Recent  Formation.  —  It  is  intended  to  embrace  in  this 
term  strata  which  have  been  formed  since  the  creation  of  man. 
It  is,  however,  impossible  to  separate  them  by  any  well-defined 
characters  from  those  of  the  tertiary  period.  The  recent  formation 
consists  of  land  which  is  forming  by  the  filling  up  of  lakes,  and  by 
the  increase  of  deltas  from  the  accumulated  sediment  which  rivers 
have  furnished. 

There  is,  however,  no  doubt  but  that  formations  on  a  large 
scale  have  continued  in  progress  over  extensive  areas  of  the  bed 
of  the  sea ;  and  they  have  been  no  less  rapid,  we  may  presume, 
than  they  were  in  earlier  periods.  But,  though  they  are  preserv- 
ing the  records  of  the  present  era,  they  will  probably  remain  in  a 
great  measure  inaccessible  for  many  ages. 

These  deposits,  so  far  as  they  are  accessible,  are  found  to  con- 
tain the  remains  of  plants  and  animals  (including  man)  now  living 
in  the  vicinity  where  the  deposits  are  forming. 


SECTION   VI. FOSSILS. 

Any  organic  substance  imbedded  in  a  geological  formation,  or 
any  product  of  organic  life,  as  a  coprolite  or  a  coin,  or  any  mark- 
ing which  an  organic  substance  has  given  to  a  rock,  is  regarded  as 
a  fossil.  The  study  of  fossils,  as  a  branch  of  practical  geology, 
requires  an  acquaintance  with  the  principles  and  the  minute 
details  of  botany  and  zoology.  Without  this  knowledge,  however, 
many  of  the  general  conclusions  to  which  the  study  of  fossils  has 
led  may  be  understood. 

1.  Fossils  are  preserved  in  differ  eiit  ways.  —  "When  any 
organic  substance  is  imbedded  in  a  forming  rock,  it  may  itself 
remain ;  or  it  may  be  removed  by  the  infiltration  of  water,  or  other 
causes,  so  gradually  as  to  leave  its  form,  and  even  its  most  delicate 


58  FOSSILS. 

markings,  in  the  rock ;  or  some  mineral  substance  may  have  been 
substituted,  and  fill  the  space  which  the  organic  substance  once 
occupied ;  that  is,  it  may  be  an  organic  substance  preserved,  it 
may  be  an  impression  of  it,  or  it  may  be  a  cast  of  it. 

2.  The  process  by  which  the  substitution  in  this  last  case  is 
effected  is  called  mineralization.     The  mineralizing  ingredient  is 
generally  derived  from  the  contiguous  rock.     In  siliceous  rocks  it 
is  si  lex.     In  calcareous  rocks  it  is  carbonate  of  lime.     When  iron 
is  diffused  through  a  rock,  it  often  becomes  the  mineralizer.     The 
substituted  mineral  is  generally  a  very  perfect  representation  of  the 
original  fossil.     We  cannot  therefore  suppose  that  the  original 
substance  was  entirely  removed  before  any  of  the  mineral  matter 
was  deposited.    The  substitution  must  have  taken  place  particle  by 
particle,  as  the  organic  matter  was  removed.     Fossils  are,  in  fact, 
often  found,  in  which  the  mineralization  has  been  arrested  after  it 
had  commenced,  so  that  the  fossil  is  in  part  an  organic  and  in  part 
a  mineral  substance.     It  has  been  proved,  by  direct  experiment, 
that  these  changes  of  removal  and  substitution  are  simultaneous. 
Pieces  of  wood  were  placed  in  a  solution  of  sulphate  of  iron. 
After  a  few  days,  the  wood  was  found  to  be  partially  mineralized, 
and  after  the  remaining  ligneous  matter  had  been  removed  by 
exposing  it  to  a  red  heat,  "  oxide  of  iron  was  found  to  have  taken 
the  form  of  the  wood  so  exactly,  that  even  the  dotted  vessels, 
peculiar  to  the  species  employed,  were  distinctly  visible  under  the 
microscope." 

3.  As  the  fossiliferous  strata  are  generally  of  marine  origin,  it  is 
to  be  presumed  that  only  a  small  proportion  of  terrestrial  animals 
are  preserved ;  and  our  knowledge  of  the  organic  remains  which 
are  preserved  is  yet  so  imperfect,  that  discoveries  are  constantly 
making,  as  examinations  are  extended.     Still,  enough  is  known  to 
enable  us  to  draw  some  satisfactory  conclusions  as  to  the  order  in 
which  living  beings  were  created  upon  the  earth. 

Though  most  of  the  earlier  organic  forms  which  have  been  pre- 
served are  of  animal  origin,  yet  vegetable  remains  occasionally 
occur  in  connection  with  them,  and  we  must  suppose  vegetables  to 


FOSSILS.  59 

have  been  produced  abundantly.  For  all  animal  food  consists  of 
vegetable  substances,  or  of  animal  substances  which  have  once 
existed  in  the  vegetable  form.  No  animal  is  capable  of  effecting 
those  combinations  of  inorganic  matter  upon  which  its  growth  and 
sustenance  depend.  We  may  therefore  conclude  that  the  intro- 
duction of  animals  and  vegetables  was  contemporaneous. 

The  greatest  development  of  vegetable  life  was,  however,  during 
the  carboniferous  period.  The  design  of  this  abundant  growth  was 
prospective.  It  was  not  produced  for  the  support  of  animal  life, 
but  for  fuel,  and  stored  till  man  should  be  introduced,  and  so  far 
advanced  in  civilization  as  to  make  this  supply  of  carbonaceous 
matter  subservient  to  his  wants  and  happiness. 

In  the  earlier  periods,  the  lower  forms  of  animal  life  were, 
beyond  all  comparison,  the  most  abundant ;  yet  the  four  great 
divisions  of  the  animal  kingdom,  Radiated,  Articulated,  Mollus- 
cous, and  Vertebrated  animals,  were  all  represented.  There  is, 
however,  no  evidence  that  any  vertebrated  animals,  except  fishes, 
were  created  till  after  the  carboniferous  period.  In  the  next 
formation,  the  new  red  sandstone,  we  find  the  tracks  of  reptiles 
and  birds,  and  probably  of  marsupial  animals.  The  first  evidence 
of  the  existence  of  mammalia  in  great  numbers  is  in  the  tertiary 
period,  when  the  pachydermata  and  edentata  were  so  much  more 
abundant  than  they  have  ever  been  since,  and  when  the  bimana 
first  appear. 

But  there  is  no  evidence  from  geology  that  man  existed  till 
after  the  close  of  the  tertiary  period.  The  grounds  ,upon  which 
contrary  statements  have  sometimes  been  made  are  untenable.  In 
Ohio  a  very  perfect  impression  of  a  human  foot  was  found  on  a 
slab  of  limestone  of  the  silurian  age.  But  it  was  subsequently 
ascertained  to  have  been  common  for  the  aborigines,  in  the  vicinity 
of  their  encampments,  to  cut  in  the  rocks,  with  surprising  accu- 
racy, the  forms  of  the  tracks  of  man  and  other  animals. 

There  is  a  human  skeleton  in  the  British  Museum  imbedded  in 
solid  limestone,  and  another  in  Paris,  both  taken  from  Guadaloupe. 
It  was  at  one  time  supposed,  from  the  degree  of  solidification  of 


60  FOSSILS. 

the  limestone,  that  it  must  have  been  formed  at  an  early  geological 
period;  but  it  is  found  that  the  beach-sand  of  that  island  now 
solidifies  rapidly,  from  the  carbonate  of  lime  which  the  waters  there 
hold  in  solution.  It  is  rendered  probable  that  the  skeletons  found 
there  have  not  been  buried  more  than  a  century  and  a  half. 

4.  As  many  parts  of  the  bed  of  the  present  seas,  which  are 
probably  receiving  detrital  matter  constantly,  are  unfavorable  for 
the  development  of  animal  life,   while  other   parts   are   highly 
favorable,  it  might  be  presumed  that  animal  life  would  be  equally 
scanty  in  particular  localities  while  the  earlier  rocks  were  form- 
ing, and  in  other  localities  very  abundant.     Hence  some  strata,  for 
hundreds  of  feet  in  thickness,  are  composed  almost  entirely  of  fos- 
sils, while  other  strata  are  nearly  or  quite  destitute  of  them.     The 
same  member  of  a  formation  may  in  one  place  be  full  of  fossils, 
and  in  another  without  them.     The  distribution  of  fossils   is 
therefore  subject  to  no  general  law ;  at  least,  none  of  which  we  can 
avail  ourselves,  in  the  search  for  them. 

5.  The  value  of  fossils  in  geology  consists  in  the  use  which  is 
made  of  them  in  determining  the  origin  and  age  of  strata. 

As  the  animal  species  which  inhabit  bodies  of  fresh  water  are 
always  different  from  those  found  in  the  sea,  their  remains  consti- 
tute the  best  means  of  determining  whether  a  formation  is  of 'fresh 
water  or  marine  origin.  In  order  to  decide  this  point,  it  may,  in 
some  cases,  be  necessary  to  be  acquainted  with  the  habits  of  par- 
ticular species.  In  most  cases,  however,  it  will  be  sufficient  to 
remember  that  in  fresh-water  formations,  first,  there  are  no 
.  44.  sponges,  corals,  or  chambered  shells ;  second, 

the  univalves  all  have  entire  mouths  (Fig. 
44).  Third,  the  bivalves  are  all  bimuscular 
(Fig.  47).  If,  therefore,  a  formation  is  found 
to  contain  sponge,  coral,  a  chambered  shell, 
a  univalve  with  a  deeply  notched  mouth  (Fig. 
45),  or  a  uriimuscular  bivalve  (Fig.  46),  it 
must  be  considered  a  marine  formation. 

We  have  seen  that  the  same  formation,  as 
exhibited  in  different  places,  differs  in  its 


FOSSILS. 


61 


thickness,  composition  and  degree  of  solidification.     If  we  could 
trace  the  strata  through  all  the  intermediate  space,  we  might  be 


Fig.  45. 


Fig.  46. 


Fig.  47. 


certain  of  their  being  the  same  formation,  notwithstanding  the 
change  in  lithological  characters.  But  this  can  seldom  be  done, 
even  for  a  few  miles  in  extent.  Sections  of  the  strata  are  obtained 
only  occasionally,  where  rivers  have  cut  through  them,  or  where, 
over  limited  areas,  the  soil  has  been  removed  from  the  outcropping 
edges.  It  is  also  frequently  the  case  that  the  strata  are  so  much 
disturbed  that  their  position  will  furnish  no  aid  in  determining  their 
age.  When  folded  axes  occur  (as  here  represented),  the  older  strata 

are  often  the  up- 
permost. There 
is  an  instance  in 
the  Alps  in  which 
strata  of  vast 
thickness  have 
been  inverted  du- 
ring the  process 
of  upheaval,  and 
now  rest  on  a 
bed  of  rock  formed  from  the  debris  which  they  had  supplied. 
6 


62  FOSSILS. 

And  yet  it  is  important  to  determine  what  formations  are  of  the 
same  age,  notwithstanding  their  displacements,  difference  in  litho- 
logical  characters,  and  separation  by  great  distances  and  by  moun- 
tains or  oceans.  This  determination  can  be  made  only  by  a  com- 
parison of  the  imbedded  fossils.  It  is  found  that  every  formation, 
and  every  important  member  of  a  formation,  contains  an  assem- 
blage of  fossils  peculiar  to  itself.  When  very  widely  separated, 
the  species  of  fossils  may  not  be  identical,  but  so  very  similar  that 
they  are  regarded  as  equivalent  species.  The  identification  of 
formations  consists  in  the  identification  of  fossils.  It  is  for  this 
purpose  mainly  that  fossils  are  regarded  as  of  so  great  importance. 

6.  If  each  formation  is  characterized  by  the  presence  of  new 
species,  it  follows  that  the  work  of  creation  was  a  progressive  one, 
continued  through  long  periods  of  time.  The  latest  creation  of  which 
we  have  any  geological  evidence  is  that  of  man.  And  if  the  lead- 
ing design  of  the  existence  of  this  earth  was  as  a  theatre  for  the 
development  of  moral  character,  it  is  to  be  presumed  that  the  work 
of  creation  ceased  when  a  species  possessing  moral  capacities  had 
been  introduced. 

It  follows  also,  from  what  has  been  said,  that  there  has  been  a 
constant  disappearance,  a  death,  of  species.  It  would  seem  that 
each  species  has  a  life  assigned  to  it,  which  is  to  be  completed  and 
surrendered.  Though  its  continuance  is  many  times  longer  than 
the  life  of  any  individual  of  the  species,  yet  it  is  the  course  of 
nature  that  species  should  disappear. 

There  may  be  something  in  the  constitution  of  each  species  by 
which  its  continuance  is  limited,  making  an  old  age  and  death 
necessary,  as  it  is  in  individuals.  But  there  are  other  causes  by  which 
the  duration  of  species  may  often  be  terminated.  The  subsidence 
of  New  Holland  would  cause  the  destruction  of  a  large  number  of 
species.  The  preservation  of  the  human  species  was  at  one  time 
effected  only  by  a  special  and  miraculous  interference.  Slowly  oper- 
ating causes  are  now  at  work,  by  which  many  species,  such  as  the 
elephant,  wolf  and  tiger,  will  at  length  become  extinct.  Their 
existence  in  a  natural  state  cannot  long  be  continued  in  a  civilized 


ANTIQUITY   OF   THE   EARTH.  63 

country.  The  forest,  their  natural  abode,  disappears,  and  some 
are  intentionally  destroyed,  because  they  render  life  and  property 
unsafe.  Under  the  operation  of  these  causes,  the  Irish  elk  (cervus 
giganteus)  has  become  extinct,  probably  within  the  human  era. 
The  Dodo,  a  gallinaceous  bird,  found  living  when  maritime  com- 
munication between  Europe  and  the  East  Indies  was  first  estab- 
lished, is  now  extinct.  The  Apteryx,  a  bird  belonging  to  New 
Zealand,  has  probably  become  extinct  since  the  commencement  of 
the  present  century. 


SECTION   VII. THE   TIME   NECESSARY   FOR   THE   FORMATION   OF  THE 

STRATIFIED   ROCKS, 

There  are  no  means  of  which  the  geologist  can  avail  himself  to 
determine  the  antiquity  of  the  earth,  or  the  amount  of  time  since 
the  sedimentary  deposits  commenced.  But  a  nigh  degree  of 
antiquity  may  yet  be  shown. 

The  materials  for  all  the  stratified  rocks  have  been  obtained 
by  the  destruction  of  previously  solidified  igneous  rocks.  This 
destruction  may  have  been  accomplished  in  part  by  the  operation 
of  volcanic  forces,  but  much  of  it  is  the  result  of  slow  disintegra- 
tion, and  of  the  eroding  power  of  running  water ;  and  we  can 
scarcely  conceive  of  a  period  sufficiently  protracted  for  such 
results. 

This  conclusion  of  the  high  antiquity  of  the  earth  is  confirmed 
by  observing  that  the  stratified  rocks'  consist  of  layers  often  not 
thicker  than  sheets  of  paper,  and  probably  not  averaging  the  tenth 
of  an  inch ;  and  yet  each  layer  is  separate  from  the  rest,  in  conse- 
quence of  some  change  in  the  conditions  under  which  it  was  depos- 
ited. Each  layer  was  probably  produced  by  the  deposition  of  all 
the  sediment  furnished  at  one  time,  and  hence  only  as  many  layers 
would  be  formed  in  a  year  as  the  number  of  freshets  in  the  rivers 
which  furnished  the  materials.  If  we  consider  the  fossiliferous  and 
metamorphic  rocks  to  be  each  forty  thousand  feet  in  thickness,  — 


64  ANTIQUITY   OF   THE   EARTH. 

which  is  not  too  large  an  estimate,  —  we  must  reckon  the  years  by 
hundreds  of  thousands  to  make  the  time  sufficiently  extended  for 
the  result. 

All  the  formations  of  any  considerable  extent  now  above  the 
surface  of  the  sea  existed  before  the  creation  of  man,  for  none 
of  them  contain  any  evidence  of  the  existence  of  human  beings ; 
and  if  they  had  existed  while  these  strata  were  forming,  sufficient 
evidence  would  have  been  left  of  the  fact,  either  in  the  form  of 
fossilized  human  bones,  or  of  works  of  human  art.  Hence,  what- 
ever be  the  estimate  which  we  form  of  the  antiquity  of  the  earth, 
from  the  slowness  of  denudation,  or  from  the  thickness  of  the 
strata,  we  must  now  add  to  that  estimate  the  period  elapsed  since 
the  creation  of  the  human  species. 

We  have  seen  that  at  different  periods  of  the  earth's  history  dif- 
ferent species  of  animals  inhabited  it.  We  are  unable  to  fix  with 
accuracy  the  ordinary  duration  of  species.  But  the  species  which 
are  now  extinct  probably  had  an  existence  as  long-continued  as 
will  be  enjoyed  by  species  now  living.  Many  recent  species  are 
known  to  have  existed  at  least  nearly  six  thousand  years,  without, 
in  most  cases,  any  indications  of  their  soon  becoming  extinct. 
Whatever  period  be  assigned  as  the  ordinary  duration  of  species, 
that  period  has  been  several  times  repeated ;  for  the  earth  has  been 
several  times  re-peopled,  and  every  time  by  species  which  had  not 
before  existed. 

Moreover,  the  amount  of  organic  matter  in  the  strata  must 
have  required  long  periods  of  time  for  its  accumulation.  The 
vegetable  deposits,  now  converted  into  coal,  are  generally  several 
feet  thick,  and  often  over  a  hundred  feet,  and  are  known  to  extend 
over  several  thousand  square  miles,  both  in  this  country  and  in 
Europe.  Many  of  the  sedimentary  rocks  consist  almost  entirely 
of  animal  remains.  The  mountain  limestone,  for  instance,  is  eight 
hundred  feet  or  more  in  thickness,  and  in  some  places  consists  of 
the  exuviae  of  encrinites  and  testacea. 

In  other  cases  the  length  of  time  required  is  shown,  not  from 
the  amount  of  organic  remains,  but  from  the  evidence  that  they  were 


ANTIQUITY   OP  THE  EARTH.  65 

deposited  very  slowly.  The  polishing  stone  called  tripoli  is  found 
in  beds  of  ten  or  twelve  feet  in  thickness,  and  is  composed  entirely 
of  the  siliceous  shells  of  animalcules,  so  minute  that,  according  to 
the  estimate  of  Ehrenberg,  the  number  in  a  cubic  inch  is  forty-one 
billions.  Several  other  rocks,  such  as  semi-opal  and  flint,  are  some- 
times found  to  have  a  similar  constitution.  The  time  necessary 
for  the  accumulation  of  beds  several  feet  thick  by  the  shells  of 
animalcules  so  minute  must  have  been  very  great. 

Each  of  these  facts  carries  us  back  to  a  period  immeasurably 
anterior  to  the  creation  of  man,  as  the  epoch  when  the  sedimentary 
deposits  commenced.  There  are  no  facts  in  geology  which  point 
to  a  different  conclusion.  It  is  of  the  utmost  importance  to  the 
geological  student  to  familiarize  himself  with  this  principle.  It  will 
assist  him  in  comprehending  the  greatness  of  geological  changes, 
and  in  applying  other  principles  in  explanation  of  geological 
phenomena. 

This  principle,  so  obvious  to  any  one  who  allows  himself  to 
reason  from  the  facts  which  geology  presents,  has  sometimes  been 
regarded  as  at  variance  with  the  Mosaic  account  of  the  creation. 
And  if  this  account  really  assigns  an  antiquity  to  the  earth  of  not 
more  than  six  thousand  years,  the  difficulty  exists. 

The  statements  made  by  Moses  are  found,  upon  examination,  to 
be  of  the  most  general  character.  They  assert,  in  the  first  place,  sim- 
ply that  "  In  the  beginning  God  created  the  heaven  and  the  earth." 
The  time  which  elapsed  after  this  first  act,  and  previously  to  the 
acts  of  creation  subsequently  recorded,  is  not  limited  by  the  sacred 
narrative.  It  may  have  been  during  this  indefinite  lapse  of  time 
that  God  gave  existence  and  enjoyment  to  a  large  number  of  ani- 
mal species  on  the  surface  of  the  earth,  and  at  the  same  time 
effected  most  of  those  physical  changes  in  the  crust  of  it  which 
have  rendered  it  a  fit  abode  for  intellectual  and  moral  beings. 

But  if  the  word  day,  in  the  first  chapter  of  Genesis,  be  consid- 
ered to  mean  a  prolonged  period  (and  philologists  regard  such  an 
interpretation  as  admissible),  then  that  chapter  is  a  record  of  the 
most  important  events  in  the  history  of  the  earth  up  to  and  includ- 


66  ANTIQUITY   OF   THE   EARTH. 

ing  the  introduction  of  man.     And  the  account,  thus  understood, 
coincides  with  the  results  of  geological  examinations. 

Instead,  then,  of  discrepancy  between  the  works  and  the  word  of 
God,  we  have  this  remarkable  fact,  that  a  history  of  the  earth, 
written  long  before  the  science  of  geology  was  known,  is  not  con- 
tradicted, but  confirmed,  by  the  progress  of  science  thus  far. 


OP  THE 

UNIVERSITY  •  J 


CHAPTER    III. 

OF  THE  CHANGES  TO  WHICH  THE  CRUST  OF  THE  EARTH 
HAS  BEEN  SUBJECTED. 

SECTION  I. CHANGES  WHICH   HATE  TAKEN  PLACE  AT  GREAT  DEPTHS 

BELOW   THE   SURFACE. 

THE  lowest  change  of  winch  we  can  gain  any  information 
is  the  formation  of  granite.  It  will  be  shown  hereafter  that 
it  has  been  in  a  melted  state,  and  that  it  has  taken  its  present 
form  on  cooling.  But  whether  any  considerable  portions  of  the 
granitic  masses,  or  of  the  melted  masses  now  below  the  surface, 
have  resulted  from  the  fusion  of  stratified  rocks,  we  have  not 
the  means  of  determining.  It  is,  however,  not  improbable,  that 
in  the  changes  of  level  to  which  the  crust  of  the  earth  has 
been  subjected,  the  stratified  rocks  may  have  gone  down  so  far 
as  to  become  melted.  At  the  same  time,  the  melted  rock  which 
is  thrown  to  the  surface  by  volcanoes  is  subjected  to  the  various 
destroying  agencies  by  which  it  becomes  sedimentary  matter,  to  be 
deposited  as  mechanical  strata.  Thus,  as  the  igneous  rocks  from 
below  are  brought  up  to  furnish  materials  for  mechanical  strata, 
there  must  be  an  equal  amount  of  depression  of  the  mechanical 
strata  towards  the  seats  of  igneous  action.  And  if  this  change 
takes  place  more  rapidly  than  the  thickness  of  crust  increases, 
then  portions  of  the  sedimentary  rocks  must  be  undergoing  fusion. 

Next  above  the  granite  an  immense  thickness  of  rock  occurs, 
which  exhibits,  from  its  stratification  and  from  the  water-worn  frag- 
ments which  it  contains,  distinct  evidence  of  its  mechanical  origin. 
And  yet  it  is  very  different  from  the  later  mechanical  forma- 


68  CHANGES    IN    STRATIFIED   ROCKS. 

tions.  It  is  more  highly  crystalline ;  it  has,  to  a  great  extent, 
assumed  a  cleavage  distinct  from  the  planes  of  stratification,  and 
chemical  affinity  has  been  so  far  active  as  to  produce  new  combina- 
tions, and  give  to  them  their  peculiar  crystalline  form,  as  in  the 
case  of  garnets,  iron  pyrites,  &c.  These  strata  also  differ  from 
those  above  them  in  containing  no  organic  remains.  It  is  not 
certain  that  organic  life  existed  on  the  earth  at  the  time  when 
these  rocks  were  deposited.  Either  it  did  not,  or  the  evidence  of 
i{  in  the  strata  of  that  period  has  been  obliterated.  The  changes 
have  at  least  been  sufficient  to  justify  their  being  characterized  as 
metamorphic  rocks. 


SECTION   II. CHANGES     IN   THE   MASS   OF    THE   STRATIFIED    ROCKS. 

1.  The  stratified  rocks  were  deposited  as  mud  or  sand,  and 
were  at  first  in  a  yielding  state.     Most  of  these  deposits  have 
become  solidified  rock,  such  as  limestone,  clay  slate  and  sand- 
stone.    The  chalk  of  England  is,  however,  but  imperfectly  consol- 
idated, the  great  sandstone  formation  of  New  Holland  is  a  friable 
mass  easily  disintegrated,  and  occasionally  beds  of  clay  in  a  plastic 
state  are  found  as  far  down  as  the  coal.     Among  the  later  rocks 
the  solidification  is  less  general,  though  there  is  some  degree  of 
hardening  in  all  except  the  most  superficial  layers.     The  fissile 
structure  results  from  the  solidification  of  the  particles  composing 
each  layer  separately. 

2.  Since  the  solidification  of  the  strata,  or  perhaps  in  connection 
with  it,  there  has  been  something  of  movement  among  the  parti- 
cles, resulting  in  mineral  veins,  conchoidal  structure,  &c.     One 
of  the  most  general  changes  of  this  kind  is  that  by  which  a  mass 
becomes  separable  into  thin  sheets,  independent  of  the  stratifica- 
tion, and  not  parallel  with  it.     This  structure  is  represented  by 
Fig.  48,  in  which  the  heavier  lines  are  those  of  stratification,  and 
the  lighter  of  cleavage. 

3.  The  strata  have  been  everywhere  more  or  less  broken,  and 


CHANGES   IN    STRATIFIED    ROCKS. 


the  fractures,  nearly  vertical,  extend  to  groat  depths.     When  a 
fracture  reaches  the  surface,  it  often  becomes  a  channel  for  water. 


Fig.  48. 


It  is  thus  widened  by  the  erosion,  the  deepest  parts  become  filled 
with  debris,  and  it  becomes  a  gorge,  ravine  or  valley. 

If  the  fracture  does  not  come  to  the  surface,  it  becomes  a 
cavern.  In  limestone,  caverns  which  are  formed  in  this  way  are 
very  frequent,  and  extend  for  many  miles.  There  is  generally  a 
stream  of  water  running  through  them,  but  not  of  sufficient  volume 
to  have  produced  the  erosion  which  has  been  effected. 

When  the  sides  of  the  fracture  are  but  little  separated,  some 
mineral  often  separates  itself  from  the  adjacent  rock,  and  filling  up 
the  space,  reunites  the  broken  parts.  It  is  then  called  a  vein  of 
segregation  (Fig.  49,  a  b).  But  the  fracture  is  more  frequently 

Kg.  49. 


filled  with  some  volcanic  rock  injected  from  below.  It  is  then  a 
dike  (c  d),  and  may  have  a  width  of  many  rods,  though  it  often 
diminishes  in  width  till  it  is  a  mere  thread.  A  dike  of  which  the 
injected  material  is  a  metallic  ore  is  a  mineral  vein. 

4.  The  uplifting  force  by  which  the  fracture  is  produced  has 
frequently  raised  the  rock  on  one  side  higher  than  it  has  on  the 
other.  This  is  called  a  fault.  (Fig.  50.)  The  unequal  move- 
ments by  which  the  fault  is  produced  seem  in  some  instances  to 
have  been  repeated  several  times,  and  the  grinding  of  the  broken 


70 


CHANGES   IN    STRATIFIED   ROCKS. 


edges  upon  each  other  has  polished  and  striated  the  sides  of  the 
fracture. 

Fig.  50. 


5.  Sedimentary  rocks  are  often  found  with  the  planes  of  their 
strata  more  or  less  inclined.  It  is  evident  that  they  were  not 
thus  formed.  The  depositions  of  sediment  from  water  will  always 
be  horizontal,  or,  at  most,  only  slightly  inclined.  But  there  is 

often  .evidence  in  the  rock  itself  that 
its  strata  were  once  horizontal.  It 
is  frequently  observed  that  vertical 
strata  contain  pebbles  with  their 
longer  axes  in  the  plane  of  the  strata. 
(Fig.  51.)  When  these  pebbles  were 
deposited,  the  longer  axes  would  take, 
on  an  obvious  mechanical  principle, 

a  horizontal  position.      Their  present  vertical  position  must  have 
resulted  from  a  change  in  the  position  of  the  strata  in  which  they 

Fig.  52. 


are  enclosed.     The  same  thing  is  shown  by  the  position  of  a  petri- 
fied forest  in  the  south  of  England,  known  as  the  Portland  dirt- 


CHANGES   IN   STRATIFIED    ROCKS. 


71 


bed.  Some  parts  of  it  are  inclined  at  an  angle  of  forty-five 
degrees.  The  position  of  the  vegetable  remains  (Fig.  52)  shows 
that  when  they  were  growing  the  surface  was  horizontal. 

The  line  b  d  (Fig.  53),  on  inclined  strata  which  makes  with  the 
horizon  the  greatest  angle,  is  called  the  direction  of  the  dip.  The 
angle  thus  formed  (a  b  d)  is  the  angle  of  inclination.  When 
inclined  strata  come  to  the  surface,  the  exposed  edge,  b  c,  is  the 
outcrop,  and  the  line  of  outcrop  on  a  horizontal  surface  is  called 
the  strike  of  the  strata. 

Fig.  53. 


When  the  inclined  position  is  produced  by  an  uplift  of  the 
strata,  along  a  given  line,  so  that  they  dip  in  opposite  directions, 
this  line  is  called  an  anticlinal  axis,  as  at  Fig.  54.  If,  however, 

Fig.  54. 


the  strata  are  fractured  along  this  line,  as  at  d,  the  fracture 
becomes  a  valley  of  elevation. 


72 


CHANGES   IN    STRATIFIED    ROCKS. 


If  depression  take  place  along  a  given  line,  as  at  c,  the  strata 
will  dip  towards  this  line,  and  it  will  be  a  synclinal  axis.  The 
depression  will  be  a  valley  of  subsidence.  A  synclinal  axis 
would  also  be  produced  by  an  elevation  of  the  strata,  as  at  d  and  e, 
on  each  side  of  it,  and  the  valley  thus  produced  is  one  of  elevation. 

When  successive  sets  of  strata,  as  /  and  d,  Fig.  53,  are  not 
parallel,  they  are  said  to  be  unconf or  viable. 

6.  When  the  strata  are  subjected  to  displacement,  they  do  not 
always  take  a  merely  inclined  position,  but  are  often  contorted 
(Fig.  55),  or  folded  together  (Fig.  56).  These  folded  axes  fre- 

Fig.  55. 


quently  succeed  each  other  for  many  miles.     (See  Figs.  7  and 

Fig.  66. 


82.)    In  the  case  represented  by  Fig.  56,  if  the  highest  portion 
has  been  removed,  so  that  the  line  a  b  represents  the  actual  sur- 


CHANGES   OP   ELEVATION   AND   SUBSIDENCE.  73 

face,  we  shall  have  apparently  a  succession  of  deposits,  of  which 
those  at  b  would  be  the  newest,  and  the  oldest  would  be  found  at 
a,  when  in  fact  the  strata  at  the  extremities  are  parts  of  the  same 
layer. 

It  is  probable  that  disturbances  like  those  now  mentioned  have 
been  taking  place  continually,  in  different  places,  from  the  earliest 
times.  There  have  been  no  periods  of  universal  disturbance,  and 
none  of  universal  repose.  On  the  contrary,  th$  periods  of  disturb- 
ance in  one  part  of  the  world  have  been  periods  of  repose  in 
another.  For  example,  the  coal  measures  of  Europe  were  much 
broken  and  disturbed  before  the  deposition  of  the  new  red  sand- 
stone, and  the  close  of  the  coal  period  was  at  one  time  supposed  to 
have  been  a  period  of  general  convulsion.  It  is  now  ascertained 
that  the  principal  coal-fields  in  this  country  were  not  much  dis- 
turbed at  that  period,  and  have  not  been  since. 


SECTION   in. CHANGES   OF   ELEVATION   AND   SUBSIDENCE. 

The  continents,  if  we  except  the  more  rugged  and  broken 
portions,  rise  from  the  sea  with  an  almost  imperceptible  ascent ; 
and  even  the  mountains  have  a  much  gentler  slope  than  we  are 
apt  to  suppose,  so  that  a  section  of  the  earth  parallel  to  the  equa- 
tor would  be  almost  a  perfect  circle.  The  slope  of  a  mountain, 
from  its  base  to  its  highest  point,  rarely  forms  with  the  horizon  an 
angle  of  as  much  as  twelve  degrees.  In  the  following  figure  (57), 
A  represents  the  peak  of  Chimborazo,  B  of  Teneriffe,  C  of  jEtna, 
and  D  of  Mount  Loa,  the  principal  volcano  of  the  Sandwich 
Islands.  The  highest  mountains  would  be  represented  on  a  twelve- 
inch  globe  by  an  altitude  of  less  than  the  one-hundredth  of  an 
inch  above  the  level  of  the  sea.  But  the  rising  and  sinking  of 
these  masses,  though  so  small  compared  with  the  dimensions  of  the 
earth,  are  yet  geological  changes  on  the  largest  scale. 

1.  The  Elevation  of  Mountains.  — •  Mountains  have  formerly 
7 


74 


ELEVATION    OF    MOUNTAINS. 


been  covered  with  the  waters  of  the  ocoan.    This  is  evident,  in  the 
case  of  some  mountains,  from  the  existence  of  stratified  rocks 


Fig.  57. 


reaching  to  the  summits.  The  stratification  could  have  been  pro- 
duced only  by  deposition  from  water.  It  is,  moreover,  evident 
from  the  existence  of  marine  fossils,  distributed  through  these 
strata,  so  abundantly,  that  they  cannot  be  accounted  for  on  any 
other  hypothesis  than  that  the  animals  lived  and  died  where  the 
remains  of  them  are  now  found.  These  strata  must  therefore  have 
formed  the  bed  of  the  sea  while  the  fossils  were  accumulating. 

There  is  no  direct  evidence  that  the  granitic  mountain  peaks 
were  ever  submerged.  But  there  is  reason  for  believing  that 
the  sedimentary  strata  which  now  occupy  the  lower  slopes  were,  at 
the  time  of  their  deposition,  continuous,  —  the  igneous  rock  having 
subsequently  broken  through  them, —  so  that  the  waters  of  the  ocean 
once  rested  on  the  whole  area  which  the  mountain  now  occupies! 

If  the  ocean  could  ever  have  been  above  its  present  level  suffi- 
ciently to  have  covered  all  the  sedimentary  rocks,  we  might 
assume  that  the  height  of  mountains  has  not  been  changed.  But 
the  level  of  the  ocean  cannot  be  subject  to  much  variation.  The 
total  amount  of  water  on  the  globe  is  always  the  same.  If  the 
continents  and  mountains  were  all  submerged  at  once,  and  the 


ELEVATION   OF   MOUNTAINS.  75 

waters  were  expanded  by  the  highest  temperature  consistent  with 
the  liquid  form,  there  would  not  be  a  change  of  level  of  more  than 
two  hundred  and  fifty  feet.  We  may  assume,  then,  that  the  ocean 
level  has  always  been  essentially  the  same  that  it  now  is.  We 
must  therefore  conclude  that  the  sedimentary  rocks,  and  the  moun- 
tains of  which  they  form  a  part,  have  been  elevated  to  their 
present  position  from  the  bed  of  the  sea. 

Different  mountain  ranges  have  been  elevated  at  different 
periods.  The  silurian  and  carboniferous  formations  were  depos- 
ited before  the  Alleghany  Mountains,  which  they  contributed  to 
form,  were  elevated ;  while  the  new  red  sandstone  and  the  creta- 
ceous and  tertiary  formations  were  deposited  subsequently  to  the 
upheaval.  They  are  accordingly  found  at  the  base  of  the  range, 
nearly  horizontal,  and  have  risen  above  the  level  of  the -ocean  only 
as  the  continent  generally  has  risen.  The  Pyrenees  were  elevated 
after  the  deposition  of  the  cretaceous  rocks,  and  have  carried  them 
up  so  that  they  appear  at  a  high  angle,  while  the  tertiary  rocks 
at  the  base  are  horizontal,  as  in  the  United  States.  The  Andes 
have  carried  up  the  tertiary  rocks  with  them,  and  their  elevation 
must  therefore  belong  to  a  recent  period.  It  appears  that  they 
are  even  yet  rising. 

It  has  recently  been  shown  that  the  Alps  have  been  subjected 
to  upheaval  at  several  distinct  periods.  At  the  close  of  the  silu- 
rian period  they  formed  a  cluster  of  islands.  At  the  commence- 
ment of  the  tertiary  period  they  became  a  mountain  range,  and 
at  the  close  of  that  period  they  were  thrown  up  some  two  thousand 
feet  higher,  to  their  present  position.  Nearly  the  same  things  will 
probably  be  found  true  of  other  mountain  ranges,  when  their  struc- 
ture has  been  minutely  studied. 

The  elevation  of  contiguous  parallel  ridges  will  necessarily  leave 
intervening  valleys  of  elevation.  As  mountain  ranges  generally 
consist  of  several  such  ridges,  valleys  of  this  description  are 
numerous,  and  they  are  often  of  great  extent. 

It  is  obvious  that  there  are  mountains  in  the  sea  of  as  great 
height  above  the  lowest  valleys  as  the  mountains  of  continents  are 


76  ELEVATION    OF   CONTINENTS. 

above  the  level  of  the  sea.  If  a  new  continent  should  hereafter 
be  formed  by  the  elevation  of  a  large  area  of  the  bed  of  the  sea, 
the  existing  mountains,  now  appearing  in  the  form  of  islands, 
would  partake  of  the  general  movement,  and  the  new  continent 
would  have  the  same  general  diversities  of  surface  as  existing  con- 
tinents. The  mountains  would  have  existed  long  before  the  con- 
tinent. It  is  therefore  to  be  supposed  that  the  mountains  of  the 
present  continents  were  elevated  before  the  continents,  and  that 
they  stood  for  long  periods  as  islands,  exposed  to  the  action  of 
waves,  tides,  and  marine  currents. 

2.  The  Elevation  of  Continents.  —  Continents  have  been  elevated 
by  so  slow  a  movement  that  it  has  not  generally  been  perceived,  even 
when  they  have  been  peopled  by  nations  advanced  in  civilization. 
And  yet  satisfactory  evidence  is  always  left  of  former  sea-levels. 

Almost  every  seaboard  furnishes  examples  of  beaches,  evidently 
once  washed  by  the  sea,  but  now  elevated  more  or  less  above  high 
water. 

At  Lubec,  near  the  northern  extremity  of  the  coast  of  Maine, 
barnacles  *  are  found  attached  to  the  rocks  eighteen  feet  above  high 
water.  The  pilots  at  that  place,  and  for  a  hundred  miles  north 
and  south  of  it,  speak  of  the  ship-channels  as  diminishing  in  depth, 
though  it  is  certain  that  they  are  not  filling  up.  Such  facts  are 
to  be  explained  only  by  supposing  that  the  coast  is  rising. 

Lakes  are  numerous  throughout  the  northern  portions  of  North 
America,  which  are  receiving  annually  large  quantities  of  sedi- 
ment, and  must  ultimately  become  alluvial  plains.  Those  of  mod- 
erate depth,  as  Lake  Erie,  cannot  require  periods  very  protracted 
to  fill  them.  Their  continuance  in  such  abundance  indicates  that 
the  elevation  of  the  continent  to  its  present  height  ~is  compara- 
tively recent.  This  conclusion  is  confirmed  by  evidence  of  another 
kind.  Throughout  this  region  of  lakes,  beds  of  clay  containing 
the  remains  of  existing  species  of  marine  animals,  are  found  at  all 
elevations  from  the  sea-coast,  to  the  height  of  about  four  hundred 
feet,  but  not  higher.  These  clay  beds  are  very  recent,  and  were 


*  The  barnacle  is  a  marine  animal,  permanently  fixed  to  the  rocks,  and 
live  but  a  short  time  without  being  surrounded  by  sea-water. 


can 


ELEVATION    OF   CONTINENTS. 


77 


deposited  when  the  surface  was  four  hundred  or  five  hundred  feet 
lower  than  it  now  is ;  and  this  amount  of  elevation  has  left  the 
existing  lakes  scattered  over  the  surface.*1 

The  following  (Fig.  58)  exhibits  Europe  as  it  was  during  the 
Fig.  58. 


Silurian  epoch,  and  Fig.  59  as  it  was  at  the  commencement  of 
the  tertiary  epoch.  The  land,  as  it  then  existed,  is  represented  by 
the  white  surface,  the  present  waters  by  the  dark  shading,  and  the 
land  which  has  been  reclaimed  from  the  ocean  by  elevation  since 
those  periods  by  the  lighter  shading. 

*  "  It  is  remarkable  that  on  the  shores  of  the  great  lakes  there  are  certain 
plants  the  proper  station  of  which  is  the  immediate  neighborhood  of  the  ocean, 
as  if  they  had  constituted  part  of  the  early  flora  of  those  regions  when  the 
lakes  were  filled  with  salt  water,  and  have  survived  the  change  that  has  taken 
place  in  the  physical  conditions  of  their  soil."  —  Torrey's  Flora  of  the  State  of 
New  York. 

7* 


78 


ELEVATION    OF    CONTINENTS. 


The  whole  southern  part  of  South  America,  embracing  an  area 
equal  to  that  of  Europe,  has  been  elevated  within  a  very  recent 
period  ;  and  some  parts  of  it,  if  not  all  of  it,  are  still  rising.  The 

Fig.  59. 


\ 


shells  found  on  the  plains  from  Brazil  to  Terra  del  Fuego,  and  on 
the  Pacific  coast,  at  a  height  of  from  one  hundred  to  thirteen  hun- 
dred feet,  are  identical  with  those  now  inhabiting  the  adjacent  seas. 
And  "  besides  the  organic  remains,  there  are,  in  very  many  parts, 
marks  of  erosion,  caves,  ancient  beaches,  sand-dunes,  and  successive 
terraces  of  gravel,"  all  which  must  have  resulted  from  the  action 
of  the  waves  at  a  period  not  remote.  At  Lima,  articles  of  human 
skill  peculiar  to  the  original  inhabitants  of  Peru  were  found  im- 
bedded in  a  mass  of  sea-shells  eighty-three  feet  above  the  present 
sea  level.  The  elevation  on  the  Pacific  coast  has  been  in  part  by 
sudden  uplifts  of  a  few  feet  at  a  time ;  but  it  is  found,  from  time 


SUBSIDENCE   OF   LAND.  79 

to  thno,  that  there  has  been  a  change  of  level,  amounting  to  a  foot 
or  more  in  a  year,  when  there  have  been  none  of  these  sudden 
movements. 

A  considerable  portion  of  Europe,  reaching  from  North  Cape  in 
Norway  to  near  the  southern  part  of  Sweden,  more  than  a  thou- 
sand miles,  and  from  the  Atlantic  to  St.  Petersburg,  more  than 
six  hundred  miles,  has  been  rising  at  the  rate  of  about  three  feet 
in  a  century,  for  at  least  two  centuries,  and  probably  much  longer. 
This  change  is  proved  by  the  occurrence,  at  considerable  elevations 
above  the  sea,  of  shells  now  found  in  the  Baltic ;  by  rocks  once 
sunken,  now  raised  above  the  surface  of  the  sea,  and  by  ancient 
seaports  having  become  inland  towns.  To  determine  the  truth  by 
actual  measurement,  the  Royal  Academy  of  Stockholm,  about 
thirty-five  years  since,  caused  marks  to  be  cut  in  the  rocks  along 
the  coast,  to  indicate  the  ordinary  level  of  the  water.  This  is 
easily  ascertained,  as  the  Baltic  is  nearly  a  tideless  sea.  The  pres- 
ent level  of  the  sea,  compared  with  that  indicated  by  the  marks 
before  mentioned,  leaves  no  doubt  that  the  country  is  rising. 

3.  The  Subsidence  of  Land.  —  Elevations  can  be  shown  to 
have  taken  place  by  fossils,  and  by  other  evidences  of  former  sea 
levels  which  are  left  on  the  surface  ;  but  depressions  leave  but  few 
indications  of  change  of  level.  It  is  yet  doubtful  whether  the 
depression  is  equal  to  the  elevation ;  that  is,  whether  the  amount 
of  land  remains  nearly  constant,  or  whether  there  has  been  an 
augmentation  of  the  dry  land  within  the  tertiary  and  recent  peri- 
ods. We  are  certain  that  the  augmentation,  if  any,  has  not  been 
equal  to  the  elevation,  for  subsidences  to  a  great  amount  are  known 
to  have  taken  place. 

There  are  occasional  instances  of  submerged  forests  seen  at  low 
tide,  at  some  distance  from  the  shore.  There  are  several  near  the 
coast  of  England  and  Scotland,  and  near  the  coast  of  Massachu- 
setts. They  are  but  a  few  feet  below  low  water,  and  do  not  indi- 
cate a  subsidence  of  more  than  about  twenty  feet. 

Numerous  instances  are  on  record  of  the  sinking  down  of 
wharfs  and  buildings  near  the  sea  during  earthquakes.  Almost 


80 


SUBSIDENCE    OF    LAND. 


Fig.  CO. 


every  violent  earthquake  is  accompanied  by  a  change  of  level.  The 
changes  of  this  kind  which  have  been  noticed  are  in  seaport  towns, 
because  greater  facilities  are  there  afforded  for  detecting  them,  and 
because  loss  of  property  awakens  attention  to  them ;  but  there  is 

every  reason  to  sup- 
pose that  these  changes 
of  level  extend  to  great 
distances  both  into  the 
country  and  into  the 
sea. 

An  immense  area 
in  the  Indian  and  Pa- 
cific Oceans,  probably 
ten  millions  of  square 
miles,  is  undergoing 
change  of  level.  The 
lines  A  B  and  D  G 
(Fig.  GO)  represent 
nearly  the  axes  of  de- 
pression ;  while  an  in- 
termediate and  two 
exterior  parallel  lines 
would  represent  axes 
of  elevation.  The  ev- 
idence of  these  changes 
is  found  principally  in 
the  peculiarities  of  the 
wall  of  coral  rock  en- 
circling the  islands. 

The  following  fig- 
ures represent,  in  sec- 
tions, modifications  of 
form  of  the  same  isl- 
and. The  coral  wall 
built  up  around  the  island  by  the  polyps,  from  the  depth  of  fifty, 


ELEVATION    AND    SUBSIDENCE.  81 

or  at  most  of  a  hundred  feet,  is  shown  at  c  c  (Fig.  61).     If  the 

Fig.  61. 


island  is  elevated,  this  wall  becomes  a  fringing  reef  (Fig.  62),  V 
becoming  the  level  of  the  sea,  and  the  animal  begins  a  new  wall 

Fig.  62. 


at  the  same  deith  as  before.  But  if  the  island  is  gradually  sink- 
ing, the  wall  is  kept  built  up  to  the  surface,  and  becomes  a  barrier 
reef  (Y\g.  63).  A  channel  is  thus  left  between  the  island  and  the 
reef,  which,  though 

jig.  Go. 

gradually  filling  up 
with  broken  coral  or 
other  sediment,  is  gen- 
erally deep  enough  for 
a  ship-channel.  If 
the  island  continue  to 
subside  till  it  disap- 
pears, and  the  coral  wall  is  still  kept  at  the  surface,  it  then  becomes 
an  atoll,  a  circular  coral  island  (Fig.  64),  often  of  many  leagues 
in  diameter,  beaten  by  the  surf  on  the  outer  edge,  but  enclosing  a 


82  ELEVATION   AND    SUBSIDENCE. 

quiet  lake,  which  communicates  only  by  occasional  channels  with 
the  ocean. 

Fig.  64. 


The  islands  contiguous  to  the  lines  A  B  and  C  D  (Fig.  60)  are 
uniformly  atolls,  or  are  surrounded  by  barrier  reefs,  and  are  there- 
fore subsiding;  while  the  islands  at  a  distance  from  these  lines 
are  surrounded  by  fringing  reefs,  which  indicate  that  they  are 
rising. 

A  well-authenticated  instance  of  gradual  subsidence  is  that  of 
Greenland.  The  entire  western  coast,  from  its  southern  extremity 
to  Disco  Island,  a  distance  of  six  hundred  miles,  has  for  the  last 
two  centuries  been  slowly  subsiding.  The  dwelling-houses  and 
places  of  worship  built  by  the  early  European  settlers  are  now  in 
part  or  entirely  submerged.  The  natives  are  said  to  be  aware  of 
the  subsidence,  and  never  build  their  huts  near  the  sea. 

4.  We  have  thus  seen  that  both  elevation  and  depression  may 
take  place.  There  is  reason  to  believe  that  these  changes  of  level 
have,  in  some  cases,  been  several  times  repeated.  In  one  of  the 
eastern  ranges  of  the  Andes,  opposite  to  Chili,  there  is  a  mass  of 
marine  strata  of  five  thousand  feet  in  thickness.  About  the  mid- 
dle of  the  series  there  occurs  a  silicified  forest.  In  one  place  a 
clump  of  coniferous  trees  was  found  of  more  than  fifty  in  number, 
and  a  foot  or  more  in  diameter.  The  base  of  the  strata  must  have 
been  twenty-five  hundred  feet  below  the  surface  of  the  sea,  in 
order  to  admit  of  the  deposition  of  the  first  half  of  it.  It  was 


ELEVATION   AND   SUBSIDENCE. 


83 


then  elevated,  so  that  a  forest  grew  upon  its  surface.  It  was  then 
depressed  at  least  twenty-five  hundred  feet,  more,  to  admit  of  the 
deposition  of  the  subsequent  strata,  and  the  whole  is  now  uplifted 
to  form  a  mountain  range  of  eight  thousand  feet  in  height. 

The  temple  of  Jupiter  Serapis,  near  Naples,  in  Italy,  was  built 
near  the  sea,  about  eighteen  hundred  years  ago.  It  was  gradually 
submerged,  and  finally  lost  by  the  deposition  of  sediment  nearly 
to  the  top  of  the  columns.  It  was  afterwards  elevated,  so  as  to  be 
entirely  above  the  level  of  the  sea.  The  remains  of  the  temple 
(Fig.  65)  were  afterwards  discovered  by  the  columns  projecting  a 

Fig.  65. 


little  above  the  ground.     The  sediment  was  removed  to  the  depth 
of  forty-six  feet,  when  the  workmen  came  to  the  base  of  the  col- 


84  ELEVATION    AND    SUBSIDENCE. 

urans,  and  to  a  pavement  seventy  feet  in  diameter.  In  1807  an 
artist  was  employed  to  take  drawings  of  the  ruin.  The  pavement 
was  then  above  the  level  of  the  sea.  Sixteen  years  afterwards  the 
same  artist  found  the  pavement  covered  with  water,  and  the  depth 
has  continued  to  increase  since  that  time.  It  is  considered  that 
for  the  last  forty  years  the  depression  has  been  three-fourths  of  an 
inch  a  year. 

Instances  enough  have  now  been  given  to  show  how  extensively 
the  system  admits  of  change.  They  are  sufficient  to  justify  us  in 
searching  for  indications  of  great  revolutions  in  past  times,  even 
where  no  such  indications  have  as  yet  been  discovered.  They  will 
serve  as  a  key  to  many  otherwise  inexplicable  phenomena,  In 
order  to  the  interpretation  of  such  phenomena  readily,  we  must 
cease  to  look  upon  these  as  exceptional  cases,  and  regard  them  not 
only  as  facts,  but  as  facts  of  frequent  occurrence. 

From  the  examples  which  have  now  been  given,  as  well  as  from 
speculations  upon  the  cause  of  these  changes,  it  seems  highly  prob- 
able that  all  the  surface  of  the  solid  portion  of  the  earth,  whether 
land  or  the  bed  of  the  sea,  is  undergoing  changes  of  level.  It  may 
be  so  gradual  that  in  the  life  of  an  individual  it  would  be  imper- 
ceptible, even  where  the  best  means  of  detecting  it  exist.  These 
means  are  generally  the  works  of  man,  and  they  are  themselves  so 
liable  to  change,  that  it  would  be  scarcely  possible  to  detect  varia- 
tions of  level,  which  amount  to  but  a  few  inches  in  a  century. 

If  we  admit  that  the  relations  of  land  and  water  have  always 
been  variable,  it  is  impossible  to  arrive  at  any  certain  conclusion  as 
to  the  amount,  position  or  form,  of  the  dry  land  at  any  former 
period.  We  may  determine,  with  some  degree  of  certainty,  what 
portions  of  the  present  continents  were  submerged  at  particular 
epochs.  Thus,  we  may  infer  that  most  of  this  country  was  sub- 
merged during  the  silurian  period,  from  the  great  extent  of  the 
Silurian  rocks ;  and,  from  the  limited  extent  of  the  chalk  formation 
in  this  country,  we  know  that  during  the  cretaceous  period  most  of 
the  continent  was  above  the  surface  of  the  sea.  But  we  have 


CUANGES   ON   TUB    SURFACE   OF   THE   EARTH.  85 

absolutely  no  data  for  determining  what  portions  of  the  bed  of  the 
sea  were  at  any  time  dry  land. 

It  is  supposable  that  the  land  has  been  principally  confined  to 
the  equatorial  regions  at  one  period,  and  to  the  polar  at  another. 
At  still  a  different  period  the  land  may  have  existed  as  islands 
scattered  through  a  general  ocean.  These  relations  may,  there- 
fore, be  assumed  to  have  existed,  if  there  are  geological  phenomena 
which  best  accord  with  such  relations. 


SECTION   IV. CHANGES    ON    THE   SURFACE   OF   THE   EARTH. 

1.  The  principal  changes  of  this  class  consist  in  the  wearing 
down  and  removing  immense  quantities  of  the  surface  rock.     The 
form  in  which  the  igneous  rocks,  of  which  the  entire  crust  of  the 
earth  was  originally  composed,  now  appear,  furnishes  no  assistance 
in  judging  of  the  amount  of  denudation  which  they  have  suffered. 
We  can  judge  only  from  the  amount  of  rock  for  which  they  have 
furnished  the  materials,  and   these  are   the  whole   sedimentary 
series  which  exist  both  as  dry  land  and  as  the  bed  of  the  sea. 

2.  The  sedimentary  rocks  have  also  been  subject  to  great  denu- 
dation ;  and  we  often  have,  in  what  is  left,  some  indications  of 
how  much  has  been  removed.     One  of  these  indications  consists  in 
the  now  level  surface  of  those  portions  of  country  in  which  large 
faults  exist.     By  the  excavations  for  coal,  in  England,  faults  have 
been  discovered  of  five  or  six  hundred  feet.     At  the  time  that  they 
were  formed,  the  surface  must  have  presented  precipitous  escarp- 
ments (as  represented  by  the  dotted  lines  in  Fig.  50)  of  a  height 
equal  to  the  dislocation ;  but  the  whole  is  now  reduced  to  a  gen- 
eral level  (z  z),  denuding  causes  having  removed  the  elevated 
portions. 

The  extent  of  valleys  will  often  give  some  idea  of  the  amount 
of  denudation  to  which  a  region  has  been  subjected.  In  the  north- 
west of  Scotland  there  is  a  succession  of  hills  of  about  three  thou- 
sand feet  in  elevation,  consisting,  for  the  upper  two  thousand  feet, 


OO  CHANGES   ON    THE    SURFACE   OF   THE   EARTH. 

of  horizontal  strata  of  old  red  sandstone.     (Fig.  66.)     We  can- 
not conceive  that  these  mountain  masses  were  deposited  in  their 

Kg.  66. 


present  isolated  form.  The  whole  intervening  spaces  must  have 
been  filled  with  strata  continuous  with  those  by  which  the  eleva- 
tions are  formed.* 

A  somewhat  similar  instance  occurs  in  the  Connecticut  river 
sandstone,  in  the  central  part  of  Massachusetts.  The  following 
figure  (Fig.  67)  represents  two  mountains  of1  the  sandstone,  between 

Fig.  67. 


which  the  Connecticut  river  flows.     The  dotted  lines  indicate  a 
depth  of  one  thousand  feet  of  the  rock  which  has  been  swept  away. 


*  "  I  entertain  little  doubt  that  when  this  loftier  portion  of  Scotland,  includ- 
ing the  entire  Highlands,  first  presented  its  broad  back  over  the  waves,  the 
upper  surface  consisted  exclusively,  from  one  extremity  to  the  other,  of  a  con- 
tinuous tract  of  old  red  sandstone  ;  though,  ere  the  land  finally  emerged,  the 
ocean  currents  of  ages  had  swept  it  away,  all  except  in  the  lower  and  last 
raised  borders,  and  in  detached  localities  where  it  still  remains,  as  in  the  pyr- 
amidal hills  of  Western  Rosshire,  to  show  the  amazing  depth  to  which  it  had 
once  overlaid  the  inferior  rocks."  —  Miller,  Old  Red  Sandstone,  p.  22. 


CHANGES    ON    THE   SURFACE   OF    THE   EARTH. 


87 


It  is  also  thought  that  a  bed  of  equal  depth  has  been  removed  from 
this  section  southward,  through  the  State  of  Connecticut,  to  the 
sea-coast. 

3.  Valleys,  and  even  many  of  the  larger  valleys,  are  produced 
by  the  wearing  down  of  the  surface.     The  lower  portion  of  the 
Connecticut  valley  is  one  of  denudation,  though  in  its  upper  part 
it  is  a  valley  of  elevation,  resulting  from  the  upheaval  of  the 
Green  and  White  Mountains.     The  water-courses  from  the  moun- 
tains are  transverse  to  the  direction  of  the  ranges,  and  generally 
consist  of  valleys  of  denudation.     These  valleys  were  no  doubt 
originally   fractures,  produced  while  the  mountains  were  rising. 
The  fractures  have  been  subsequently  widened  by  denudation  into 
valleys. 

4.  The  rocky  surface,  beyond  the  fortieth  parallels  of  latitude, 
and  in    the   vicinity   of   glacier-producing    mountains,  is  gener- 

Fig.  68.  ally  covered  with 

grooves  and  strice 
(Fig.  6 8),  varying 
from  several  inches 
in  depth  to  the 
finest  perceptible 
lines.  Rocks  that 
are  of  a  soft  con- 
sistence, or  which 
have  been  long 
exposed  to  atmos- 
pheric agents,  sel- 
dom exhibit  these 
marks,  though  there  are  probably  few  places,  outside  of  the  par- 
allels before  mentioned,  where  the  rocky  surface,  if  it  has  been 
protected  from  atmospheric  decay,  does  not  contain  such  grooving. 

5.  Another  change  at  the  surface  consists  in  the  formation  of  a 
soil;  that  is,  of  a  superficial  layer,  of  no  great  thickness,  of  earthy 
matter,  a  large  proportion  of  which  is  always  in  a  minutely  divided 
state.     In  some  instances  it  is  common  sediment,  unsolidified  ;  in 


88  CHANGES    OF    CLIMATE. 

others,  it  consists  of  the  surface  rock  in  a  state  of  disintegration ; 
but  a  large  part  of  the  soil  within  the  region  where  the  grooved 
surfaces  are  found  consists  of  materials  transported  from  a 
distance. 

Soils  are  distinguished  according  to  their  predominant  minerals, 
as  siliceous,  aluminous  and  calcareous.  If  siliceous  matter  is  in 
excess,  it  will  be  a  light,  warm  soil,  and  allow  the  water  to  pass 
through  it  too  freely.  If  the  clay  predominates,  the  soil  is  cold, 
stiff,  and  too  retentive  of  moisture.  A  proper  admixture  of  these 
three  ingredients  constitutes  the  best  soils.  There  are  some  other 
mineral  ingredients  essential  to  the  productiveness  of -soils,  but 
they  are  always  in  small  proportion.  In  addition  to  the  inorganic 
part  which  is  common  to  the  upper  soil,  and  the  subsoil,  there  is 
required,  in  order  to  render  the  upper  layer  productive,  a  large 
admixture  of  decaying  animal  and  vegetable  matter. 


SECTION    V. CHANGES    OF    CLIMATE. 

Our  means  of  determining  the  climate  of  any  former  period 
consists  in  a  comparison  of  the  fossils  of  such  period  with  the  exist- 
ing forms  of  life  in  warm  and  cold  climates. 

The  earliest  abundant  vegetation  consisted  principally  of  ferns, 
rushes  and  mosses,  and  a  larger  growth  was  attained  than  is  at- 
tained by  any  of  the  allied  forms  of  the  present  time.  We  may 
infer  that  the  circumstances  under  which  these  lower  forms  of 
vegetable  life  are  now  produced  in  the  largest  proportion,  compared 
with  other  forms,  and  under  which  they  grow  to  the  largest  size,  are 
the  circumstances  approaching  most  nearly  those  under  which  the 
early  vegetation  was  produced.  These  circumstances  are  found 
to  be  a  position  elevated  but  little  above  the  level  of  the  sea,  a 
humid  atmosphere,  and  the  highest  terrestrial  temperature.  Such 
facts  favor  the  conclusion  that  during  the  coal  period  an  ultra- 
tropical  climate  prevailed,  and  that  the  land  existed  in  the  form 
of  low  islands,  thickly  set  in  a  general  ocean. 


CHANGES    OF    CLIMATE.  89 

The  peculiar  characters  of  some  of  the  animal  fos\ils,  from  the 
earliest  fossiliferous  to  the  tertiary  series,  indicate  that  a  warmer 
climate  prevailed  during  their  formation  than  now  exists.  The 
remains  of  marine  animals,  such  as  the  cephalopoda,  are  found  in 
great  numbers  and  in  high  latitudes,  in  a  fossil  state ;  but  similar 
species,  as  the  nautilus,  now  abound  enly  between  the  tropics.  The 
same  is  true  of  the  crinoidea.  Coralline  limestone  is  also  found  in 
great  abundance  and  in  high  northern  latitudes ;  but  the  stone-pro- 
ducing coral  now  exists  only  in  very  warm  seas.  The  remains  of 
saurian  reptiles  are  numerous  in  the  oolite  and  Wealden  ;  but  all  the 
larger  recent  species  of  the  lizard  tribe,  such  as  the  crocodile,  are 
confined  to  the  warmer  regions  of  the  earth. 

A  former  warm  climate  in  Siberia  is  indicated  by  the  occur- 
rence there  of  the  remains  of  elephants.  These  animals  were  so 
abundant  that  their  tusks  are  now  collected  as  an  article  of  com- 
merce. The  abundance  and  high  state  of  preservation  of  these 
remains  seem  to  preclude  the  explanation  that  they  were  conveyed 
there,  from  the  present  tropical  regions,  by  any  great  geological 
convulsion.  The  species  must  therefore  have  inhabited  the  country, 
though  the  elephant  is  now  found  only  between  the  tropics.  The 
Siberian  elephant  was  a  different  species  from  any  now  existing, 
and,  unlike  the  recent  species,  had  a  covering  of  coarse  hair. 
There  is,  however,  no  reason  to  conclude  that  it  could  endure  a 
continued  low  temperature ;  and  its  sustenance  would  have  been 
impossible,  from  the  very  stinted  vegetation  which  that  region  now 
affords.  We  must  therefore  suppose  that  Siberia  enjoyed,  at  the 
period  when  it  supported  these  animals  in  such  abundance,  a  trop- 
ical climate. 

Most  of  the  facts  which  go  to  prove  a  change  of  climate  have 
been  observed  in  the  northern  hemisphere ;  but  the  explorations  in 
South  America  and  New  Holland  furnish  ground  for  believing  that 
the  geological  phenomena  of  the  two  hemispheres  are  essentially 
alike,  and  that  the  indications  of  climate  are  the  same  for  the  same 
periods. 

Such  is,  in  general,  the  evidence  in  reference  to  climate ;  and  it 


90  CHANGES    OF    CLIMATE. 

leads  to  the  conclusion  that  a  highly  tropical  climate  prevailed  in 
the  temperate,  and  for  some  distance,  at  least,  into  the  polar 
zones,  in  the  early  geological  periods ;  while  there  is  no  reason  for 
supposing  that  the  tropical  regions  experienced  a  temperature  too 
high  for  physical  life  to  endure  it.  The  climate  of  the  earth  was 
characterized  then  by  a  higher  temperature  than  now,  and  by 
greater  uniformity.  This  was  the  climate,  with  perhaps  a  gradual 
reduction  of  temperature,  till  the  later  portions  of  the  tertiary 
period. 

Before  the  close  of  the  tertiary  period,  a  change  occurred,  and 
probably  a  rapid  one,  to  a  more  rigorous  climate  than  now  exists. 
The  destruction  of  the  elephant  in  Siberia  was  evidently  sudden, 
and  was  followed  by  extreme  cold ;  for  the  animals  are  in  some 
cases  entirely  preserved  in  ice,  and  in  so  perfect  a  state  that,  when 
the  ice  which  surrounds  them  becomes  melted,  the  flesh  is  devoured 
by  carnivorous  animals.  There  are  occasionally  found,  in  the  drift 
of  the  boulder  period,  shells  similar  to  those  of  the  Arctic  regions, 
and  in  a  condition  to  show  that  they  have  not  been  transported. 
The  clay  beds  of  the  northern  portion  of  the  United  States  and  of 
Canada  were  deposited  during  the  last  depression  of  that  portion 
of  the  continent,  and  they  contain  the  remains  of  marine  animals 
identical  in  several  instances  with  species  now  living,  but  confined 
to  more  northern  regions.  It  must  therefore  be  admitted  that  the 
interval  between  the  middle  tertiary  and  the  modern  era  was  one 
of  great  cold.  It  is  generally  referred  to  as  the  Glacial  period. 

Very  considerable  local  changes  of  climate  have  also  occurred 
within  the  historical  period.  Thus  the  mean  temperature  of  the 
Alps  has  been  so  reduced  that  the  ancient  passes  have  in  modern 
t iiiies  become  choked  up  with  snow,  and  other  passes  have  been 
sought, —  a  result,  perhaps,  of  additional  upheaval.  It  would  seem 
that  Siberia  is  now  receiving  a  milder  climate.  The  ice  in  which 
elephants  have  for  centuries  been  imbedded  has  been  slowly  melt- 
ing for  at  least  thirty  years. 


ADVANTAGES   RESULTING    FROM    GEOLOGICAL   CHANGES. 


91 


SECTION   VI. ADVANTAGES   RESULTING   FROM  GEOLOGICAL  CHANGES. 

1.  The  division  of  the  general  surface  into  land  and  water,  as  well 
as  the  diversified  form  of  the  land,  the  existence  of  mountains  and 
low  lands,  and  the  consequent  modifications  of  climate,  the  water- 
falls, and  the  river-systems,  constituting  the  drainage  of  continents, 
are  all  results  of  the  process  of  upheaval. 

2.  A  large  part  of  the  mineral  substances  employed  for  archi- 
tectural and  economical  purposes  are  oceanic  deposits,  —  such  as  the 
marbles,  slates,  sandstones  and  mineral  salt,  —  and  would  have  been 
inaccessible  if  they  had  not  been  elevated  from  the  position  in 
which  they  were  formed.     And  the  elevation  of  them  above  the 
bed  of  the  sea  would  have  exposed  only  the  superficial  layer,  if 
they  had  not  been  either  irregularly  uplifted,  as  at  e  c  (Fig.  69), 
or  unequally  worn  down,  as  at  b. 

.       Fig.  69. 


The  granitic  rocks,  as  they  were  formed  below  the  aqueous 
rocks,  must  have  remained  unknown  and  useless,  if  they  had  not 
been  brought  to  the  surface,  as  at  c,  by  the  most  convulsive  efforts 
of  nature  of  which  we  have  any  knowledge.  Thus,  natural 
mechanical  forces  have  effected  for  man  what  the  mechanical  forces 
under  his  control  would  be  entirely  insufficient  to  accomplish. 

3.  It  is  by  changes  of  this  kind  that  we  become  acquainted  with 


92  ADVANTAGES    RESULTING    FROM    GEOLOGICAL    CHANGES. 

the  geological  structure  of  the  crust  of  the  earth.  Mining  opera- 
tions have  never  extended  to  a  greater  depth  than  three  thou- 
sand feet,  while  the  inclined  position  of  the  strata  exposes  for 
examination,  along  their  outcropping  edges,  e  a  c,  the  whole  series, 
even  to  the  primary  rocks.  The  upheaval  of  the  granitic  rocks, 
and  the  removal  by  denudation  of  the  overlying  deposits,  shows 
us  the  crystalline  character  which  the  earthy  materials  take, 
when  subjected  to  pressure  and  cooled  from  fusion  with  extreme 
slowness.  Thus  we  have,  exposed  to  observation,  the  process  of 
nature  in  the  formation  and  modification  of  rocks  for  several  miles 
in  depth.  Of  the  central  portions,  however,  including  by  far  the 
largest  part  of  the  mass  of  the  earth,  we  have  no  knowledge 
whatever. 

4.  Springs,  and  the  other  means  of  obtaining  water  for  domes- 
tic purposes,  depend  in  part  upon  the  inclined  position  of  strata, 
and  the  broken  and  uneven  condition  of  the  surface,  and  in  part 
upon  the  alternation  of  permeable  and  impermeable  strata.     If  all 
the  strata  were  porous,  like  the  sandstones,  the  water  which  falls 
upon  the  surface  would  gradually  settle  through  them  to  the  level 
of  the  sea ;  or,  if  they  were  all  impermeable,  like  the  clays,  the 
water  would  pass  over  the  surface,  and  be  collected  in  lakes  or  the 
ocean.     As  it  is,  the  porous  structure  of  the  soil  and  of  some  rocks 
acts  as  a  reservoir,  from  which  the  water  is  gradually  discharged, 
and  the  intervention  of  impermeable  strata  prevents  its  taking  a 
perpendicular  direction  downwards.    Thus,  if  the  stratum  e  b  (Fi^. 
69)  consists  of  porous  rock,  and  the  one  below  is  impermeable,  the 
water  which  is  absorbed  at  e  will  appear  at  b  as  a  spring.     Or, 
if  the  line  a  d  is  a  fracture,  the  water  received  at  c  may  reappear 
as  a  spring  at  a.     If  the  strata  were  perforated  by  boring  at  e  till 
the  porous  stratum  a  is  reached,  the  water  will  rise  to  the  sur- 
face, constituting  an  Artesian  well.     An  ordinary  well  consists  of 
an  excavation  continued  till  a  stratum  is  reached  which  is  perma- 
nently saturated  with  water. 

5.  Most  of  the  metallic  ores  which  occur  in  the  stratified  rocks, 
with  the  exception  of  iron,  are  found  in  fractures  or  as  dikes. 


ADVANTAGES   RESULTING   FROM    GEOLOGICAL   CHANGES.          93 

Without  these  disturbances  of  the  strata,  the  ores  would  have 
remained  either  sparingly  diffused  throughout  the  adjacent  strata, 
or  as  a  part  of  the  melted  mass  at  the  volcanic  centres.  The  ores 
and  metals  which  are  found  in  the  primary  rocks  are  accessible 
only  by  tl^e  bringing  up  of  these  rocks  to  the  surface. 

The  fracturing,  displacement,  and  elevation  of  the  strata, 
attended,  as  is  often  the  case,  with  the  destruction  of  property  and 
of  the  life  both  of  man  and  the  inferior  animals,  might,  at  first 
view,  be  thought  an  unnecessary,  if  not  a  wanton  infringement  upon 
arrangements  already  established.  But  the  results  which  we  have 
noticed,  though  by  no  means  a  full  enumeration  of  the  advantages 
resulting  from  geological  changes,  are  sufficient  to  show  that  even 
the  more  violent  disturbances  to  which  the  crust  of  the  earth  has 
been  subjected  constitute  an  important  part  of  that  series  of  adjust- 
ments which  has  rendered  it  a  suitable  abode  for  human  beings. 
These  changes  are  therefore  neither  useless  nor  accidental,  but  are 
essential  parts  of  a  wise  and  beneficent  system. 


CHAPTER    IV. 

OF   THE  CAUSES  OF   GEOLOGICAL  PHENOMENA. 

AN  exhibition  of  the  composition  and  structure  of  the  earth, 
together  with  an  account,  as  far  as  there  is  reliable  evidence,  of 
the  modifications  which  they  have  undergone,  has  been  the  object 
of  the  preceding  chapters.  They  are  mainly  a  collection  arid  clas- 
sification of  observed  facts.  No  reference  has  been  made  to 
causes  or  modes  of  operation,  except  in  a  few  cases  where  it  was 
necessary  in  order  that  a  statement  or  description,  might  be 
intelligible. 

If  the  facts  have  been  given  with  sufficient  clearness  and  detail 
to  convey  a  correct  general  idea  of  the  crust  of  the  earth,  we  are 
prepared  to  inquire  what  are  the  agencies  employed,  and  how  they 
have  operated  in  producing  it.  It  is  the  province  of  the  geologist 
to  question  every  known  power  in  nature,  and  to  ascertain  what 
geological  effects  each  one  is  now  producing ;  and,  observing  what 
effects  are  produced  by  given  causes,  he  is  to  judge  of  the  causes 
which  have  produced  like  effects  in  past  geological  periods. 

Some  of  these  causes  are  in  their  nature  limited,  and  effects  can 
be  referred  to  them  only  within  those  limits.  Thus,  the  congela- 
tion of  water  expands  it  by  a  certain  proportion  of  its  volume,  and 
beyond  that  it  can  have  no  effect.  But  the  expansive  power  of 
steam  varies  with  the  temperature ;  and  hence  the  effects  referred 
to  it  may  be  equally  varied.  Thus,  we  are  not  to  expect  exact 
uniformity  of  results  in  all  past  times,  but  the  results  will  vary 
only  as  the  circumstances  vary  upon  which  the  operation  of  these 
causes  depends. 


ATMOSPHERIC   CAUSES.  95 

Geological  causes,  in  most  instances,  operate  with  extreme  slow- 
ness ;  and  therefore  it  will  require  a  series  of  observations,  continued 
for  a  long  time,  to  ascertain  what  are  the  capabilities  of  these 
causes.  But  a  single  instance  of  their  effects  proves  their  capa- 
bilities thus  far.  Hence,  one  instance  of  the  deposition  of  a 
stratum  of  salt  in  a  salt  lake ;  of  the  filling  of  a  fracture  with  fluid 
lava  ;  of  a  volcanic  eruption,  like  that  of  Iceland  in  1783  ;  of  the 
subsidence  of  a  volcanic  mountain,  as  that  of  Papandayang  in 
Java ;  or  of  the  rising  of  a  large  area  of  land,  as  in  Sweden,  as 
fully  proves  that  natural  causes  exist  capable  of  producing  these 
effec-ts,  as  if  the  effects  were  produced  daily.  As  these  effects 
increase  in  number,  and  careful  observations  are  made  and  authen- 
tic accounts  preserved,  the  means  of  correctly  explaining  geologi- 
cal phenomena  will  increase.  The  causes  thus  far  known  are 
Atmospheric  Causes,  Chemical  Action,  Organic  Agency,  and 
Aqueous,  Aqueo-glacial  and  Igneous  Action. 


-    SECTION    I. ATMOSPHERIC    CAUSES. 

The  oxygen  of  the  atmosphere  is  capable  of  uniting  with  some 
of  the  constituents  of  rocks,  by  which  their  cohesion  is  weakened  or 
destroyed.  This  is  the  cause  of  the  rapid  disintegration  of  some 
varieties  of  granite.  The  protoxide  of  iron  which  they  contain  is 
converted,  by  contact  with  the  atmosphere,  into  the  peroxide.  Its 
volume  is  thus  increased,  and  portions  of  the  rock  are  separated 
from  the  mass.  When  granite  or  limestone  contains  sulphuret  of 
iron,  the  oxygen  of  the  atmosphere,  in  connection  with  moisture, 
combines  with  the  sulphur,  forming  sulphuric  acid,  by  which  lime- 
stone and  the  felspar  of  granite  are  rapidly  decomposed.  Hence, 
a  rock  which  contains  an  oxide  or  sulphuret  of  iron  should  not  be 
used  for  architectural  purposes. 

Carbonic  acid  is  another  constituent  of  the  atmosphere  which 
operates  as  a  decomposing  agent.  The  water  that  falls  from  the 
atmosphere  is  charged  with  it,  and  thus  becomes  capable  of  dis- 


96  ATMOSPHERIC    CAUSES. 

solving  calcareous  rocks.  Carbonic  acid  is  thus  indirectly  the 
means  of  the  rapid  destruction  of  rocks  of  this  class.  It  is  also 
believed  that  carbonic  acid  enters  into  direct  combination  with 
some  of  the  constituents  of  rocks,  and  particularly  felspar ;  for  it  is 
found  that  in  those  countries  where  carbonic  acid  issues  in  great 
quantities  from  the  earth,  the  rocks,  especially  those  which  contain 
felspar,  disintegrate  rapidly.  Masses  of  many  tons'  weight,  which 
appear  to  be  solid  granite,  after  being  broken  are  found  to  be  in 
such  a  state  of  decay  that  fragments  may  be  reduced  to  sand 
between  the  fingers. 

The  moisture  of  the  atmosphere  has  some  effect  as  a  decompos- 
ing agent.  Rocks  which  are  exposed  to  frequent  alternations  of 
moisture  and  dryness  soon  crumble  into  fragments.  Eain,  falling 
upon  the  surface  of  rock,  produces,  mechanically,  a  destroying 
effect,  which  is  not  to  be  overlooked. 

Variations  of  temperature,  especially  those  alternations  above 
and  below  the  freezing  point,  have  greater  influence  than  any 
other  cause  in  the  destruction  of  rocks.  When  the  water  with 
which  a  rock  is  saturated  congeals,  the  resulting  expansion  tends 
to  enlarge  the  interstices,  and  thus  to  separate  the  particles  of  the 
rock.  When  the  ice  melts,  the  particles  fail  to  resume  the  close- 
ness of  arrangement  with  which  they  were  before  packed.  By 
frequent  repetition  of  this  action,  the  superficial  portion  loses  its 
cohesion,  and  disintegrates.  It  is  also  found  that  in  the  region  of 
perpetual  snow  the  surface  of  the  mountain  masses  is  covered  with 
rock  in  a  disintegrated  or  fragmentary  state,  in  greater  abundance 
than  below  the  snow  line ;  but  no  explanation  of  this  fact  has  yet 
been  found. 

In  mountainous  regions,  electrical  discharges  and  violent  storms 
have  some  destroying  effect.  Winds  have  considerable  power 
in  changing  the  place  of  earthy  matter  in  a  disintegrated  state. 
In  deserts,  the  sands  are  carried  in  great  quantities  to  great 
distances. 

The  causes  now  enumerated,  when  considered  separately,  and  as 
acting  for  only  limited  periods  of  time,  seem  hardly  worthy  of 


CHEMICAL   ACTION.  97 

notice ;  but  when  considered  as  operating  conjointly,  and  for  indefi- 
nite periods  of  time,  they  must  have  produced  important  changes 
on  the  surface  of  the  earth. 

From  these  causes,  the  surface  and  ornaments  of  castles  and 
other  ancient  edifices,  and  of  boulders,  and  all  insulated  rocks, 
are  found  to  be  decayed,  and  often  to  a  considerable  depth.  It  is 
from  these  causes  that  a  soil  is  produced  on  every  surface  of  rock 
which  is  not  so  exposed  to  the  action  of  currents  that  the  debris  is 
removed  as  fast  as  it  is  formed.  Hence  it  is,  also,  that  a  slope  of 
detritus  is  formed  at  the  base  of  every  declivity,  so  that  the  ledge 
appears  only  at  the  highest  points. 

It  is  from  a  combination  of  these  atmospheric  causes  that  a  large 
part  of  the  sediment  is  furnished  which  brooks  and  rivers  carry 
away.  And  when  cohesion  is  not  entirely  overcome,  it  is  so  far 
weakened  that  other  causes  are  much  more  effectual  than  they 
would  otherwise  be,  in  effecting  the  disintegration  of  rocks. 


SECTION   II.  —  CHEMICAL  ACTION. 

All  those  changes  in  which  the  action  is  molecular,  —  that  is,  be- 
tween the  molecules  as  such,  and  not  between  the  masses,  —  includ- 
ing the  effects  of  the  imponderable  substances,  we  regard  as  result- 
ing from  chemical  agency. 

Under  the  control  of  these  molecular  forces  the  crystalline  rocks 
have  taken  their  form ;  and  if  the  crust  of  the  earth  could  have 
remained  in  a  fixed  condition,  in  which  these  forces  would  have 
been  in  equilibrium,  no  further  chemical  action  could  have  taken 
place.  But,  instead  of  being  in  a  fixed  condition,  the  present  sys- 
tem is  one  of  perpetual  change.  Various  disturbances  of  this 
equilibrium  of  forces, — such,  for  instance,  as  the  diurnal  and  annual 
changes  of  temperature  at  the  surface,  and  the  still  greater  secular 
changes  of  temperature  at  great  depths,  —  will  bring  the  chemical 
forces  into  operation.  The  mechanical  disintegration  of  the  crys- 
talline rocks,  and  the  deposition  of  them  in  strata  independently  of 
9 


98  CHEMICAL   ACTION. 

the  chemical  affinity  of  their  particles,  will  give  occasion  for 
chemical  changes,  —  that  is,  for  a  rearrangement  of  the  particles  in 
accordance  with  their  affinities,  —  whenever  any  movement  of  the 
particles-  among  themselves  can  take  place.  These  movements  take 
place,  to  a  very  great  extent,  under  the  influence  of  electrical  cur- 
rents, and  of  change  of  temperature,  even  while  the  masses  retain 
their  solid  form. 

Chemical  affinity  has  exhibited  itself  on  the  largest  scale  in  the 
formation  of  the  various  mineral  species  of  which  the  crust  of  the 
earth  is  composed ;  but  we  may  also  refer  to  the  same  cause  the 
formation  of  divisional  planes  in  rocks,  the  concretionary  arrange- 
ment, and  mineral  veins. 

1.  Divisional  Planes. —  It  has  before  been  stated,  that  the  older 
rocks,  in  many  cases,  cleave  freely  in  planes  not  parallel  with  the 
stratification.  (See  Fig.  48.)  In  some  instances,  in  beds  of  lava, 
a  similar  cleavage  exists,  sufficiently  perfect  to  allow  of  its  use  as 
a  roofing  material.  In  these  cases,  there  must  have  been  a  rear- 
rangement of  the  particles,  so  that  their  axes  of  greatest  attraction 
would  lie  in  parallel  planes ;  the  same  arrangement  which  exists  in 
mica  and  other  crystalline  substances,  which  have  one  and  but  one 
free  cleavage. 

A  similar  arrangement  has  sometimes  taken  place  under  such 
circumstances  as  to  submit  the  process  to  more  careful  scrutiny. 
In  the  gold  mines  of  Chili,  the  powder  from  which  the  gold  has 
been  washed  is  "  thrown  into  a  common  heap.  A  great  deal  of 
chemical  action  then  commences ;  salts  of  various  kinds  effloresce 
on  the  surface,  and  the  mass  becomes  hard,  and  divides  into  frag- 
ments which  possess  an  even  and  well-defined  slaty  structure" 
When  a  portion  of  clay,  worked  into  a  paste  with  a  very  weak 
acid,  is  submitted  to  a  weak  voltaic  action  for  several  months,  and 
then  dried,  it  is  found  to  have  acquired  a  distinct  though  imper- 
fect cleavage  structure. 

It  appears,  then,  that  both  electrical  currents  and  ordinary 
chemical  action  are  capable  of  arranging  the  particles  of  an  earthy 
mass  into  separable  layers.  We  may  then  regard  this  change  in  the 


CHEMICAL  ACTION.  99 

older  rocks  as  an  imperfect  crystallization,  and  probably  induced 
by  electro-chemical  agency. 

It  is  also  found  that  all  rocks  are  divided  into  huge  blocks  by 
seams  not  parallel  with  the  cleavage,  and  too  regular  to  be  consid- 
ered as  fractures.  These  seams  bear  an  analogy  to  the  secondary 
faces  of  crystals,  which  are  never  parallel  to  the  cleavage. 

2.  Concretionary  Formations.  —  There  exist  in  many  rocks 
concretions  which  differ  from  the  mass  of  the  rocks.  In  most  of 
the  tertiary  clays  there  are  small  concretionary  nodules,  which 
contain  more  calcareous  matter  than  the  mass  of  clay  around  them. 
In  the  coal  formation,  the  nodular  iron  ore  consists  of  concretionary 
masses.  In  the  chalk  formation,  nodules  of  flint  abound,  and  gen- 
erally in  layers.  In  many  of  these  cases,  particularly  in  the 
clays  and  coal,  the  nodules  have  an  organic  nucleus,  and,  although 
concretionary,  they  retain  the  marks  of  stratification  of  the  adja- 
cent rocks.  Hence  they  could  not  have  been  deposited  in  the  form 
of  nodules.  There  must  therefore  have  been  in  the  rock,  though 
in  the  solid  state,  such  motion  among  the  molecules  that  particles 
of  a  particular  mineral  have  separated  from  the  mass  and  rear- 
ranged themselves  in  concretionary  layers,  yet  so  gradually  as  not 
to  disturb  the  lines  of  original  stratification. 

There  are  other  instances,  similar  to  the  last  in  all  respects, 
except  that  the  segregated  portion  does  not  take  the  concretionary 
form.  When  gypsum  is  distributed  in  small  proportion  through 
a  formation,  there  seems  very  little  reason  to  doubt  but  that  it  is, 
by  a  molecular  action,  segregated  from  the  strata  in  lenticular 
Fig.  70.  masses,  as  at  a  (Fig.  70).  Many 

of  the  limestone  strata  contain 
irregular  aggregations  of  quartz. 
It  is  presumed  that  the  siliceous 
and  calcareous  matter  was  deposited 
together  as  sediment,  and  that  the  aggregation  has  resulted  from 
a  movement  among  the  particles  similar  to  that  by  which  the  con- 
cretionary structure  is  produced. 

The  columnar  structure  of  basalt  seems  to  have  resulted  from 


100  CHEMICAL  ACTION. 

a  peculiar  molecular  action,  at  first  resembling  a  concretionary 
arrangement,  while  the  mass  was  cooling  from  a  state  of  fusion.  In 
experimenting  to  ascertain  the  cause  of  this  structure,  Mr.  Watt 
fused  in  a  furnace  seven  hundred  pounds  of  basalt.  When  cooled, 
Fig.  71.  he  found  that  "  numerous  spheroids  had  been 

formed,  and  that  when  two  of  them  came 
in  contact,  they  did  not  penetrate  each  other, 
but  were  mutually  compressed  and  separated 
by  a  well-defined  plane,  invested  with  a 
rusty  coating.  When  several  met,  they 
formed  prisms."  (Fig.  71.) 
3.  Mineral  Veins.  —  The  phenomena  of  veins  are  such  that 
they  cannot  all  be  referred  to  the  same  cause.  In  some,  the  vein- 
stuff  has  been  protruded  as  a  dike,  differing  from  ordinary  dikes 
only  in  the  accidental  circumstance  that  it  contains  a  metal  or 
a  metallic  ore. 

Mineral  veins  are  not,  however,  generally  filled  by  injection  from 
below.  It  is  found  that  those  veins  only  are  productive  which  have 
an  east  and  west  direction.  But  injected  dikes  run  in  all  directions. 
The  ore  often  varies  in  richness  at  different  depths  in  the  vein, 
or  passes  into  ore  of  some  other  metal.  The  ore  also  varies  in 
kind  and  quality,  according  to  the  character  of  the  rock  through 
which  the  vein  passes.  These  phenomena  are  best  explained  by 
supposing  that  the  sediment  of  which  the  strata  were  formed  con- 
tained the  mineral  substances  of  these  veins  in  small  proportion. 
After  they  were  solidified,  and  fractures  had  been  formed,  the  min- 
eral substance  was  transferred  by  molecular  action  to  the  fissures, 
and  deposited. 

It  was  shown  by  the  early  experiments  of  Davy,  that  voltaic 
currents  are  capable  of  taking  up  mineral  substances  from  their 
solutions,  and  removing  them  from  one  cup  to  another.  It  has 
been  ascertained  that  in  most  mineral  veins  a  proper  apparatus 
will  detect  the  existence  of  electric  currents.  It  may  be  regarded 
as  certain,  that  the  unequal  heating  of  different  parts  of  the  sur- 
face at  the  same  time,  by  the  sun,  causes  a  vast  current  of  feeble 


ORGANIC   CAUSES.  101 

intensity  to  circulate  around  the  earth  once  in  twenty-four  hours. 
The  unequal  distribution  of  heat  below  the  surface  may  also  pro- 
duce currents  subject  to  other  laws.  We  should  expect  that  these 
currents  would  take  up  the  mineral  substances  diffused  through 
rocks,  and  deposit  them  by  themselves.  It  seems  probable,  there- 
fore, that  the  molecular  action,  from  which  the  segregation  of 
metallic  veins  has  resulted,  was  that  of  voltaic  currents.  • 


SECTION   III. ORGANIC   CAUSES. 

The  effects  of  all  organic  causes  in  producing  geological  changes 
are  inconsiderable,  compared  with  those  of  inorganic  causes.  With 
the  exception  of  the  coral  formation,  the  most  important  of  these 
effects  are  those  produced  by  human  agency.  We  find  examples 
of  this  agency  in  the  distribution  of  animals  and  plants  beyond  the 
regions  where  they  are  indigenous ;  in  the  increased  numbers  of 
certain  species,  and  in  the  diminution,  if  not  extinction,  of  others  ; 
in  the  modifications  of  climate,  dependent  on  the  destruction  of 
the  forests  and  the  cultivation  of  the  soil ;  in  controlling  the  course 
of  rivers  ;  in  arresting  by  embankments  the  encroachments  of  the 
sea ;  in  breaking  up  and  changing  the  place  of  great  quantities  of 
rock  by  mining  and  engineering  operations ;  and  in  the  increased 
quantity  of  sediment  furnished  to  streams  by  cultivating  the  ^sur- 
face, and  thus  preventing  the  protecting  influence  which  the  matted 
roots  of  trees  and  the  smaller  vegetables  would  otherwise  have. 
Such  effects,  though  attributable  mainly  to  man,  are  produced  in 
some  degree  by  all  other  animals. 

Besides  these  general  effects,  it  is  the  existence  of  organic  forms 
that  has  conferred  on  all  the  sedimentary  rocks  their  fossiliferous 
character.  The  records  of  the  climate  of  each  geological  period, 
of  the  physical  geography,  of  the  vegetable  productions,  and  of 
the  animal  forms  by  which  the  earth  was  peopled,  consist  in  the 
remains  of  the  living  beings  of  these  several  periods,  imbedded  in 
the  contemporaneous  rock  formations.  But  in  the  sediment 


102  ORGANIC    CAUSES. 

deposited  since  the  human  era  there  must  have  been  furnished 
both  the  remains  of  human  beings  and  works  of  art,  such  as 
implements  of  labor  and  war,  pottery,  coins,  fragments  of  ships,  &c. 

Moreover,  the  quantity  of  material  which  has  been  furnished  by 
organic  causes  is  by  no  means  small.  The  coal-beds  are  the  prod- 
uct of  vegetable  growth  exclusively.  We  not  unfrequently  find 
strata  of  great  extent  consisting  almost  entirely  of  the  shells  of 
molluscous  animals,  of  the  stems  of  encrinites,  or  of  the  shields 
of  microscopic  animalcules. 

But  the  most  abundant  rock  which  can  be  regarded  as  the  prod- 
uct of  animal  organization  is  the  coral  formation.  It  consists  of 
immense  walls  of  coral  limestone,  separating  either  an  atoll  or  the 
land  of  an  island  or  continent  from  the  open  sea.  The  base  of 
this  wall  has  a  width  varying  from  a  hundred  feet  to  a  mile  or 
more,  and  the  outer  edge  of  it  is  at  such  a  distance  from  the  shore 
as  to  give  a  depth  not  much  exceeding  a  hundred  feet.  Over 
this  area  of  the  bed  of  the  sea,  which'  forms  the  base  of  the  wall, 
the  coral  polyp  commenced  its  work.  Attaching  itself  in  im- 
mense numbers  over  this  area,  it  deposits  calcareous  matter  from 
its  under  surface,  and  thus,  by  degrees,  elevates  itself  towards  the 
surface  of  the  water,  till  it  reaches  a  level  a  little  above  low-water 
mark.  The  height  of  the  wall  would  not,  with  these  conditions, 
exceed  one  hundred  feet ;  but  some  hundreds  of  the  islands  sur- 
rounded by  coral  walls  are  gradually  subsiding.  The  depositions 
of  the  polyps  keep  pace  with  the  subsidence,  so  that  this  wall  has 
reached  an  elevation  from  its  base  of  a  thousand  feet,  and  in  one 
instance  of  two  thousand  feet.  (See  Figs.  61,  62,  63,  64.) 

Most  of  the  islands  of  the  torrid  zone  are  thus  surrounded  with 
coral  reefs,  except  a  few  where  the  cold  polar  currents  reduce  the 
temperature  too  low  to  admit  of  their  growth.  In  one  instance, 
along  the  north-east  coast  of  New  Holland,  there  is  a  coral  reef, 
some  twenty-five  miles  from  the  land,  which  has  a  continuous  ex- 
tension, excepting  occasional  inlets  of  no  great  depth,  of  a  thousand 
miles.  The  reef  along  the  island  of  New  Caledonia  is  four  hun- 
dred miles  long.  A  large  number  of  other  reefs  have  a  nearly 


f) 


AQUEOUS   CAUSES.  103 


equal  extension.  There  is  thus  an  area  of  several  thousands 
of  square  miles  covered  to  a  great  depth  with  this  coralline  lime- 
stone. Some  limestone  formations  of  great  extent  among  tke  older 
rocks  were  the  work  of  similar  animals.  These  lower  forms  of 
organization  have,  therefore,  always  been  important  geological 
agents,  both  in  collecting  the  carbonate  of  lime  from  its  solution 
in  the  waters  of  the  ocean,  and  in  depositing  it  as  solid  rock.  ••£' 


SECTION   IV. —  AQUEOUS   CAUSES. 

Water  is,  next  to  heat,  the  most  important  geological  agent. 
All  the  stratified  rocks  are  aqueous  deposits,  and  their  total 
amount  is  in  some  respects  a  measure  of  the  influence  which  this 
agent  has  exerted.  The  materials  have  been  obtained  from  the 
destruction  of  preexisting  rocks,  transported  by  water,  and  depos- 
ited in  layers. 

When  the  first  strata  were  formed,  the  sediment  must  have 
been  obtained  entirely  from  igneous  rocks,  because  only  those 
rocks  existed ;  but  now  it  is  obtained  from  every  kind  of  rock 
which  is  exposed  to  abrading  or  decomposing  agencies.  Hence, 
many  of  the  later  formations  contain  fragments,  and  sometimes 
within  the  fragments  well-characterized  fossils,  of  earlier  formations. 

The  sediment  which  is  ultimately  to  become  stratified  rock  is 
deposited  on  the  beds  of  the  ocean,  and  other  great  reservoirs  of 
water.  The  formation  of  most  of  the  aqueous  rocks,  therefore,  as 
well  as  of  the  igneous  rocks,  is  deep  below  the  surface  ;  and  neither 
of  these  operations,  on  the  large  scale,  is  directly  exposed  to  our 
observation.  We  may,  however,  learn  by  observation,  how  the 
sediment  is  furnished  to  the  waters  and  transported  by  them,  and 
we  can  form  some  correct  ideas  of  the  manner  in  which  it  will  be 
laid  down  on  the  bed  of  the  ocean,  and  solidified. 

I.   The  Furnishing  of  Sediment. 

1.  Almost  all  the  minerals  which  occur  in  the  geological  forma- 
tions are,  to  some  slight  extent,  soluble  in  water.  Hence,  rain 


104  AQUEOUS   CAUSES. 

water,  by  passing  through  a  stratum  of  earth  or  rock  and  reap- 
pearing as  a  spring,  loses  the  insipidity  which  it  had  as  pure 
water,*  and  becomes  palatable.  It  is  then  found  to  hold  in 
solution  some  small  proportion  of  earthy  substances,  upon  which 
this  change  of  taste  depends.  Although  the  proportion  of  dissolved 
matter  is  very  small,  yet  the  surface  of  earth  upon  which  this  dis- 
tilled water  is  shed  is  one-fourth  of  the  surface  of  the  globe,  and 
solution  below  all  that  surface  is  constantly  taking  place.  No 
inconsiderable  amount  must  thus  have  been  furnished,  from  the 
existing  rocks  of  each  period,  towards  the  formation  of  the  strata 
of  a  later  period. 

There  are  some  substances  which  are  soluble  in  water,  in  large 
quantities.  Rock-salt  is  an  example.  It  is  not  found  in  any 
very  large  proportion  in  rocks  generally,  but  a  very  large  aggre- 
gate amount  has  been  taken  up  by  the  waters  which  have  filtered 
through  the  strata.  The  ocean  gathers  into  itself,  by  degrees,  all 
the  soluble  substances  which  are  thus  taken  up.  It  receives  sup- 
plies of  water  charged  with  these  substances  from  springs,  rivers 
and  lakes.  It  returns  as  much  water  as  it  receives ;  but  it  is 
always  in  the  form  of  vapor,  and  is  therefore  pure  water.  Hence 
the  saline  properties  of  the  ocean,  and  of  those  inland  seas  which 
have  no  outlets.  There  is  thus  gathered  the  materials  for  the 
rock-salt  deposits. 

But  many  substances  which  are  not  considered  soluble  in  water 
become  so  by  some  modification  of  the  water.  Water  of  a  high 
temperature  is  capable  of  dissolving  silex.  In  Iceland  and  other 
volcanic  regions,  the  hot  springs  are  charged  with  silex,  which  is 
deposited  as  the  water  cools.  Thus,  siliceous  formations  accumulate 
around  springs  of  this  kind.  The  various  agates  may  have  been 
deposited  from  such  solutions. 

In  the  decomposition  of  mica,  felspar  and  volcanic  rocks,  a  con- 
siderable amount  of  potassa  is  set  free.  Potassa  or  soda  renders 
the  water  in  which  it  is  dissolved  capable  of  dissolving  silex  in 
large  quantity.  In  these  ways  water  removes,  with  some  degree 
of  rapidity,  one  of  the  most  insoluble  minerals  which  rocks  contain. 


AQUEOUS   CAUSES. 


105 


In  volcanic  countries,  and  in  coal  districts,  carbonic  acid  is 
abundant,  both  in  spring-water  and  in  the  gaseous  form.  Water 
charged  with  this  gas  becomes  capable  of  dissolving  limestone. 
Where  the  water  is  exposed  to  the  air,  the  gas  gradually  escapes, 
and  the  calcareous  matter  is  deposited.  Many  accumulations  of 
this  kind  are  now  taking  pkce.  Some  have  already  extended 
several  miles  in  length,  and  they  are  often  of  great  thickness, — 
in  one  instance,  in  Italy,  two  hundred  feet  (Fig.  72).  It  is  also 

Fig.  72. 


probable  that  many  calcareous  springs  issue  below  the  surface  of 
lakes  and  seas,  and  thus,  both  fresh-water  and  marine  deposits 
would  now  be  forming.  These  formations  are  distinctly  stratified, 
and  are  white  and  crystalline,  and  become  solid  at  the  time  of 
deposition. 

These  dissolved  materials  are  less  observed  than  others,  because 
they  do  not  render  the  water  turbid;  but  there  is  reason  to  believe 
that  several  of  the  aqueous  formations,  particularly  the  limestones, 
have  been  built  up  chiefly  from  them. 

2.  The  abrading  action  of  rivers  furnishes  considerable  detrital 
matter.  The  general  form  of  the  river  courses  is  determined  by 
other  causes  than  the  agency  of  the  river  itself,  yet  a  river  which 
has  a  rapid  current  is  continually  deepening  its  channel.  We  have 


106  AQUEOUS   CAUSES. 

proof  of  this  by  observing,  when  the  water  is  low,  that  irregularity 
of  surface  which  running  water  always  produces,  by  wearing  away 
the  softer  parts  of  the  rock,  and  leaving  the  harder  in  relief. 
Hence,  a  river  will  have  its  rapids  either  where  the  hardest  strata 
occur,  and  which  therefore  wear  down  least  rapidly,  or  where  the 
rock  has  been  hardened  by  the  intrusion  or  near  proximity  of 
dikes. 

The  abrading  power  of  rivers  occasionally  becomes  greatly 
increased  by  water-falls.  The  force  which  the  water  acquires  in 
its  descent  is  such  as  to  excavate  a  deep  cavity  at  the  foot  of  the 
fall,  reaching  back  under  the  ledge  from  which  the  water  descends. 
The  ledge  is  therefore  constantly  being  undermined.  The  cataract 
of  Niagara  is  peculiar,  in  having  the  rock  at  its  base  of  a  soft  and 
friable  texture,  so  that  it  is  rapidly  worn  away,  while  the  upper 
rock  is  a  compact  siliceous  limestone.  If  the  order  of  super- 
position had  been  the  reverse,  the  falls  would  have  been  converted 
into  a  series  of  rapids.  It  is  now  preserved  as  a  single  fall,  and 
as  such  it  has  probably  cut  the  gorge,  about  two  hundred  feet 
deep  and  seven  miles  in  length,  through  which  its  waters  now 
reach  Lake  Ontario.  A  few  years  since,  a  large  mass,  perhaps 
half  an  acre  in  area,  fell  from  the  centre  of  the  horse-shoe  fall. 
Another  mass  of  equal  size  has  recently  fallen  from  the  western 
extremity  of  the  ledge.  Thus  the  fall  is  gradually  receding. 

But  the  foreign  substances,  such  as  drift-wood,  ice,  sand  and 
gravel,  with  which  the  waters  of  a  river  are  occasionally  charged, 
contribute  more  than  everything  else  to  its  abrading  power.  At 
such  times  its  volume  is  generally  greatest,  and  its  current  the 
most  rapid.  Its  bed  is  then  sometimes  perceptibly  deepened  and 
widened  in  a  few  hours. 

Much  the  greater  part,  however,  of  the  earthy  matter  which 
rivers  convey  in  such  quantity  to  the  ocean,  is  furnished  by  other 
means  than  the  eroding  action  of  the  river  itself.  It  is  the  loose 
material,  the  soil  and  alluvium,  to  which  the  solid  rocks  have  been 
reduced  by  the  imperceptible  but  incessantly  operating  atmos- 
pheric agencies,  from  which  most  of  the  sediment  of  rivers  is 


AQUEOUS   CAUSES.  107 

obtained.  After  a  rain,  every  tributary  rivulet  is  turbid  with 
suspended  earthy  matter,  and  it  is  from  these  sources  that  the 
larger  streams  receive  the  most  of  their  sediment. 

Some  observations  have  been  made  for  the  purpose  of  ascer- 
taining the  quantity  of  sediment  which  rivers  annually  carry  into 
the  sea.  The  Kennebec  furnishes  materials  which,  if  spread 
evenly  on  an  area  of  one  mile  square,  and  consolidated  into  rock 
of  the  specific  gravity  of  granite,  would  have  a  thickness  of  six 
inches.  The  Merrimac  furnishes  about  two-thirds  as  much,  the 
Ganges  about  two  hundred  and  fifty  times  as  much,  and  the  Mis- 
sissippi two  thousand  times  as  much. 

Thus,  the  tendency  is,  to  reduce  the  highest  parts  of  the  land, 
and  to  fill  up  the  depressions  of  the  sea ;  and  though  we  have  not 
data  enough  to  form  any  reliable  estimate  of  the  total  annual  dis- 
charge of  sediment  into  the  ocean  by  rivers,  yet  they  are  suffi- 
cient to  show  that  the  effects  of  this  kind  are  on  a  large  scale,  and 
to  relieve  us  from  any  impression  that  existing  agencies  are  inade- 
quate to  the  production  of  the  stratified  rocks. 

3.  The  action  of  waves  is  another  means  by  which  detrital 
matter  is  furnished.  Wherever  the  shore  consists  of  loose  mate- 
rials, and  is  favorably  situated  to  be  acted  upon  by  the  waves,  there 
is  annually  a  sensible  encroachment  of  the  sea.  Such  encroach- 
ments are  rapidly  making  in  many  places;  and  thus  a  large 
amount  of  sediment  is  delivered  to  the  waters  of  the  ocean. 

The  waves  also  encroach  upon  the  coast  when  it  consists  of 
rocks,  even  of  the  most  indestructible  kinds.  They  continually 
beat  upon  it,  undermine  the  cliffs,  and  precipitate  them  into  the 
sea.  The  tides  increase  the  power  of  the  waves,  by  varying  the 
place  of  their  action,  so  as  to  present  the  same  surface  of  rock 
alternately  to  the  action  of  water  and  of  the  air,  frost  and  sun. 
During  storms,  the  waves  have  sufficient  force  to  break  off  frag- 
ments of  rock  from  the  escarpment,  sometimes  in  masses  weighing 
twenty  tons  or  more,  and  remove  them  many  rods  inland. 

A  bold,  rocky  coast  always  exhibits  evidence  of  a  great  amount 
of  erosion.  The  steep  escarpments  and  the  high  rugged  shafts  of 


108 


AQUEOUS    CAUSES. 


rock  (Fig.  73)  against  which  the  waves  now  beat  are  the  rem- 
nants of  masses  of  rock  which  once  extended  further  into  the  sea, 
but  have  been  worn  away  by  the  waves.  It  is  by  such  agency 
that  the  deep  inlets  and  harbors  of  the  coast  of  New  England  and 
Nova  Scotia  have  been  excavated. 


Fig.  73. 


This  more  violent  action  of  the  waves  is  only  occasional ;  but 
when  of  less  power,  they  are  incessantly  rolling  the  loosened  frag- 
ments of  rock  upon  each  other,  and  thus  wearing  them  down  to 
particles  small  enough  to  be  carried  away  by  the  water. 

4.  The  action  of  waves  is  confined  to  the  coast,  and  never 
extends  to  great  depths.  But  marine  currents  act  principally  on 
the  bed  of  the  sea.  The  temperature  of  the  mass  of  the  ocean 
is  much  higher  in  the  equatorial  than  in  the  polar  regions. 
At  the  surface,  the  difference  amounts  to  sixty  degrees.  The 
waters  of  the  torrid  zone  are  thus  expanded,  and  flow  over  the 
colder  waters  of  the  north  and  south  ;  while  these  colder  waters 
of  the  polar  seas  flow  back,  in  an  under  current,  towards  the 
equator. 

For  the  same  reason, —  a  difference  of  temperature, —  there  will 
be,  in  the  higher  regions  of  the  atmosphere,  a  current  of  warm  and 


AQUEOUS   CAUSES.  109 

moist  air  flowing  from  the  equator  north  and  south,  while  the  cold 
and  dry  air  conies  in  from  the  polar  regions  towards  the  equator. 
In  this  way  the  equatorial  waters  are  carried,  in  a  state  of  vapor, 
towards  the  poles,  where  they  are  condensed,  and  go  to  increase 
the  currents  of  water  moving  towards  the  equator. 

Such  are  the  general  causes  of  the  oceanic  movements  in  a 
north  and  south  direction;  but  these  currents  at  once  become 
deflected  westward,  by  the  diurnal  revolution  of  the  earth,  as  the 
trade  winds  do.  Hence  there  results  a  Pacific  equatorial  current, 
which  has  a  motion  of  about  thirty  miles  a  day,  and  an  Atlantic 
equatorial  current,  moving  from  sixty  to  seventy  miles  a  day.  The 
principal  marine  currents  are  shown  in  Fig.  74. 

The  currents  moving  towards  the  poles  are  superficial,  and 
therefore  do  not  produce  any  marked  geological  effects.  But  the 
polar  currents,  and  those  which  are  produced  from  them,  are  of 
great  depth,  and  there  is  no  reason  to  suppose  that  they  do  not 
move,  from  their  commencement,  along  the  bed  of  the  ocean. 
There  is  also  reason  to  suppose  that  they  exist  at  great  depths, 
where  the  opposing  superficial  currents  entirely  conceal  them. 

AVherever  these  currents  come  to  the  surface,  their  motion  is 
undoubtedly  greater  than  it  is  at  the  bottom,  where  it  is  retarded 
by  the  friction  which  the  moving  waters  encounter,  and  by  the 
irregularities  of  the  bed  of  the  ocean.  It  should,  however,  be 
remembered,  that  they  move  with  the  weight  of  the  whole  superior 
body  of  water ;  and  therefore,  though  the  motion  be  very  slow,  it 
will  still  possess  great  power. 

Any  irregularities  in  the  bed  of  the  ocean  beneath  such  a 
current  must  be  subject  to  very  rapid  abrasion.  We  shall  sea 
hereafter,  that  earthquake  vibrations  often  shiver  the  rocks  at  the 
solid  surface ;  and  if  any  of  these  ridges  at  the  bottom  of  the 
ocean  were  thus  acted  upon,  the  loosened  portions  would  be  swept 
away  by  the  current  and  deposited  at  lower  levels,  or  where  the 
current  subsides.  If,  in  any  instance  during  an  earthquake  con- 
vulsion, a  fault  should  be  produced  across  one  of  these  marine  cur- 
rents, like  the  great  fault  of  over  five  hundred  feet  in  England, 
10 


110 


AQUEOUS    CAUSES. 


AQUEOUS   CAUSES.  Ill 

the  abutment  thus  thrown  up  would  soon  be  worn  down ;  and  if-  it 
consisted  of  unconsolidated  matter,  it  would  be  swept  away  almost 
bodily. 

The  effect  of  such  currents  will  be  greatest  where  they  are 
deflected  by  a  continent  or  island.  Thus,  a  marine  current  sets 
from  near  New  Holland  in  a  direct  line  to  the  north  of  the  island 
of  Madagascar,  where  it  is  arrested  by  the  African  coast,  and 
deflected  into  the  narrow  Mozambique  channel,  and  there  acquires 
a  velocity  of  four  or  five  miles  an  hour.  It  is  impossible  that  any 
kind  of  rock  should  receive  the  constant  force  of  such  a  body  of 
water  without  being  rapidly  worn  away ;  and,  if  there  should  be 
any  difference  of  texture  in  this  rocky  barrier,  the  softer  portions 
would  yield  the  most  rapidly,  and  thus  valleys  might  be  formed. 

It  is  not  improbable  that  the  deep  indentation  on  the  western 
coast  of  Africa  may  have  been  due,  in  a  great  measure,  to  the  coast 
current  from  the  Cape  of  Good  Hope ;  and  that  the  Caribbean  Sea 
and  the  Gulf  of  Mexico  may  have  been  excavated  by  the  force  of 
the  Atlantic  equatorial  current  being  thrown  into  this  angle. 

We  may  regard  these  currents  as  oceanic  rivers ;  and  it  is.  obvi- 
ous that  the  volume  of  the  terrestrial  rivers  would  bear  no  com- 
parison with  that  of  these  currents,  and  their  effects  would  be 
equally  small  in  the  comparison.  The  Gulf  Stream,  and  the 
Mozambique  and  other  similar  currents,  must  be  wearing  down  the 
valleys  through  which  they  flow,  to  such  an  extent  as  to  furnish 
an  immense  amount  of  detrital  matter  for  the  formation  of  new 
rocks. 

It  is  principally  to  the  agency  of  these  deep  marine  currents 
that  we  are  to  refer  those  extensive  denudations,  so  abundant  on 
the  present  continents,  such  as  the  wearing  out  of  the  inter- 
mediate masses  of  rock  between  the  hills  already  referred  to 
(Fig.  66),  the  denudation  of  the  Connecticut  river  sandstone,  and, 
perhaps,  the  excavations  which  have  formed  Lake  Erie  and  Lake 
Ontario. 

II.    The  Transportation  of  Sediment. 

The  detrital  matter  obtained  in  these  several  ways  is  swept 


112  AQUEOUS   CAUSES. 

away  by  running  water.  The  specific  gravity  of  rocks  does  not, 
in  general,  exceed  two  and  a  half.  Hence,  to  keep  them  suspended 
in  water,  will  require  a  force  of  only  three-fifths  of  what  would  be 
necessary  to  suspend  them  in  the  atmosphere.  In  the  case  of  river 
currents,  the  velocity  and  irregularity  of  motion  are  generally 
sufficient  to  keep  all  the  finer  sediment  equally  distributed. 

There  will,  however,  be  a  division  of  the  sediment  according  to 
the  strength  of  the  currrent.  Hence,  the  bed  of  a  mountain 
stream,  if  there  is  any  loose  material,  always  consists  of  pebbles. 
As  it  approaches  the  alluvial  region,  the  bed  is  sandy ;  and  when 
the  current  becomes  very  sluggish,  it  consists  of  a  fine  mud. 

Rivers  never  deposit  all  their  sediment,  some  of  them  none  of  it, 
along  their  course.  Large  rivers  continue  partially  distinct  from 
the  ocean  water  to  a  considerable  distance  beyond  their  mouths. 
The  waters  of  the  Amazon  have  been  recognized  at  a  distance  of 
three  hundred  miles.  This  depends  in  part  upon  the  volume  and 
velocity  of  the  river ;  more,  however,  upon  the  fact  that  river  water 
is  lighter  than  sea  water.  This  extension  of  a  river  will,  in  most 
cases,  be  sufficient  to  deliver  a  part  of  its  sediment  into  a  marine 
current.  When  such  a  current  sweeps  very  near  the  mouth  of  a 
river,  as  it  does  to  that  of  the  Niger,  the  Amazon,  or  the  Missis- 
sippi, it  is  probable  that  most  of  its  sediment  is  carried  away  by  it. 

The  transporting  power  of  a  marine  current  is  greater  than 
that  of  a  river,  in  consequence  of  the  greater  specific  gravity  of  its 
water ;  but  it  has  scarcely  any  of  that  irregular .  motion  of  rapid 
rivers,  upon  which  their  transporting  power  in  a  great  degree 
depends.  The  force  of  the  current  alone,  when  it  reaches  the 
bottom,  is,  however,  sufficient  to  remove  every  form  of  loose  earthy 
matter.  Thus  it  may  be  presumed  that  the  Gulf  Stream  sweeps 
all  the  sediment  from  its  bed  until  it  reaches  the  latitude  of  Cape 
Hatteras,  where  the  cold  waters  from  the  north  begin  to  underlie 
it,  and  it  takes  the  character  of  a  surface  stream. 

But  the  transporting  power  of  marine  currents  depends  mostly 
upon  the  depth  of  water.  It  is  found,  by  experiment,  that  ordinary 
river  sediment  will  sink  in  water  about  one  foot  in  an  hour.  A 


AQUEOUS   CAUSES.  113 

current,  therefore,  of  a  thousand  feet  in  depth,  which  moves  a  mile 
in  an  hour,  would  carry  its  sediment  a  thousand  miles.  It  is  obvi- 
ous, then,  that  there  is  no  part  of  the  bed  of  the  sea  which  may  not 
be  receiving  sediment. 

III.    The  Deposition  of  Sediment. 

From  what  has  been  said  of  the  weight  of  sediment,  it  follows 
that  it  will  be  deposited  whenever  the  water  in  which  it  is 
suspended  is  at  rest.  Hence,  when  a  river  increases  in  breadth  so 
as  to  form  a  lake,  the  waters  at  the  outlet  are  seldom  turbid.  The 
earthy  matters  with  which  the  principal  and  tributary  streams 
were  charged  all  settle  to  the  bottom,  and  go  to  lessen  the  capacity 
of  the  reservoir.  Thus  lakes  are  continually  diminishing  in  depth 
and  area.  In  many  instances,  they  are  already  filled  with  sediment, 
and  are  thus  converted  into  alluvial  plains,  through  which  the 
river  flows  in  a  narrow  channel. 

It  is  frequently  the  case  that  a  river,  as  it  approaches  the  sea, 
has  so  slow  a  motion  that  its  sediment  is  deposited  on  the  bed  of 
the  stream.  Thus  the  bed  will  be  raised,  and  the  banks  will  also 
be  raised,  by  the  deposition  of  sediment  upon  them  at  periods  of 
overflow.  The  river  will  then  be  raised  above  the  adjacent  coun- 
try. The  river  Po,  for  the  last  part  of  its  course,  is  from  ten  to 
twenty  feet  above  the  adjacent  lands.  The  same  is  true  of  the 
Mississippi,  and  many  other  rivers.  The  streets  of  New  Orleans 
are  several  feet  below  the  surface  of  the  river.  In  an  uninhabited 
country,  such  a  river  would  soon  seek  a  new  and  lower  channel ;  but 
in  a  populous  country,  it  becomes  a  matter  of  interest  and  safety  to 
confine  the  river  in  its  old  channel,  by  artificial  embankments. 

But  the  principal  part  of  the  sediment  of  rivers  is  conveyed  to 
the  sea.  It  here  mingles  with  the  debris  which  the  waves  have 
furnished,  and  a  part  of  it  is  deposited  to  form  deltas.  The 
remaining  part  is  taken  up  by  marine  currents,  mingled  with  the 
debris  which  they  have  furnished,  and  is  spread  out  on  the  bed  of 
the  ocean. 

Of  the  extent  of  these  deposits  we  can  form  no  estimate.     Those 
of  rivers  and  lakes  are  comparatively  unimportant,  as  they  are  in 
' 


114  AQUEOUS   CAUSES. 

the  older  formations.  Some  of  the  delta  deposits  are  already  of 
great  extent.  That  of  the  Ganges  contains  an  area  of  twenty-six 
thousand  square  miles,  that  of  the  Niger  twenty-five  thousand,  and 
that  of  the  Nile  twelve  thousand.  The  delta  of  the  Rhone  has 
increased  its  area  by  three  hundred  square  miles  in  the  last  thou- 
sand years.  The  Po  has  encroached  upon  the  Adriatic  two  thou- 
sand square  miles  in  the  last  two  thousand  years,  and  the  Missis- 
sippi has  enlarged  its  delta  by  one  hundred  square  miles  in  the  last 
hundred  years.  In  the  deep  valleys  of  the  ocean  accumulations 
may  be  taking  place  on  as  large  a  scale  as  they  ever  have  been  in 
former  times. 

IV.    Character  of  the  Formations  thus  produced. 

Sedimentary  matter  thus  deposited  would  take  the  form  of 
strata.  Thus,  a  delta  deposit  may  receive  at  one  time  from  a 
river  a  layer  of  coarse  gravel  and  pebbles,  and  in  the  course  of  a 
few  hours  the  current  may  be  so  reduced  that  it  will  convey  to 
the  same  place  only  fine  sand  and  silt.  Or,  if  a  depositing  current 
receive  its  sediment  only  at  intervals,  the  heaviest  particles  would 
be  thrown  down  first,  and  the  more  finely  levigated  particles  would 
continue  to  fall,  till  the  water  became  transparent.  Another  supply 
would  furnish  another  similar  stratum,  and  so  on.  The  same 
arrangement  might  result  from  the  sediment  being  furnished  by 
different  rivers.  Thus,  if  sediment  were  furnished  to  the  Gulf 
Stream  by  the  Merrimac  river,  and  the  streams  emptying  into  the 
Bay  of  Fundy,  the  freshets  would  occur  earlier  in  the  season  in 
the  Merrimac,  and  it  would  furnish  a  supply  of  sediment  from  a 
region  of  primary  rocks.  A  later  supply  would  come  from  the  red 
sandstone  region  of  Nova  Scotia,  and  the  stratification  would  be 
indicated  by  the  different  kinds  of  rock  produced.  Thus  stratifica- 
tion will  result  from  difference  in  the  color,  composition,  or  size  of 
the  particles  of  which  rocks  consist.  A  great  variety  of  causes, 
both  general  and  local,  may  therefore  give  to  a  deposit  this  char- 
acter. Hence,  as  stratified  rocks  are  produced  by  the  sediment  now 
laid  down  from  water,  we  may  conclude  that  the  older  stratified 
rocks  are  the  sediment  deposited  in  like  manner,  in  former  times. 


i    UNIVERSITY  ) 

V  OF     ,  VK^X 

AQUEOUS   CAUSES.  115 

The  occurrence  of  layers  of  different  composition,  as  one  way  in 
which  the  stratification  is  indicated,  is  produced  by  local  and  fre- 
quently recurring  causes.  There  are,  however,  other  alternations 
of  much  greater  extent ;  those,  for  example,  nearly  twenty  in  num- 
ber, distinguished  by  striking  differences  in  lithological  character, 
into  which  the  New  York  system  of  rocks  is  divided.  These 
alternations  have  resulted  from  more  general  causes.  The  physical 
geography  of  a  wide  region  must  have  been  so  different,  at  the 
different  periods  during  which  these  several  formations  were 
deposited,  as  to  change,  at  each  period,  the  kind  of  sediment  fur- 
nished to  the  forming  currents,  and  modify  the  types  of  animal  life. 

We  have  seen  that  the  same  causes  that  determined  the  strati- 
fied arrangement  will  determine  the  alternations  of  strata  of  coarse 
and  fine  materials.  - 

It  is  obvious  that  the  stratification  of  the  marine  deposits  will  be 
nearly  horizontal.  If  the  surface  were  very  irregular  upon  which 
the  deposition  commenced,  the  irregularity  would  constantly  dimin- 
ish ;  for  the  movement  of  the  water  over  this  surface,  however  slow, 
would  tend  to  remove  the  accumulations  from  the  highest  points, 
and  leave  them  at  the  lowest  (Fig.  75).  Delta  and  lake  deposits 
will,  however,  dip  somewhat,  though  Fig.  75. 

never  at  a  high  angle,  towards  the 
deep  water.  In  certain  situations, 
where  a  river  and  a  tidal  wave,  com- 
ing in  conflict,  cause,  in  succession,  eddies  and  currents  in  opposite 
directions,  we  should  expect  to  find  the  stratification  very  irregular 
(Fig.  76) ;  sometimes  false  stratifications  (a  b),  sometimes  the  strata 
cut  off  abruptly,  and  at  other  times  contorted  or  dipping  in  opposite 
directions  within  short  distances. 

Wherever  sediment  is  deposited,  it  will  entomb  whatever  of  the 
remains  of  animal  or  vegetable  life  may  be  mingled  with  it.  They 
will  be  at  once  protected  against  the  influence  of  all  the  ordinary 
decomposing  agencies,  and  will  continue  for  ages  to  retain  their 
peculiar  markings,  and  even  their  colors.  They  will  thus  constitute, 
in  all  future  time,  a  record  of  the  present  condition  of  the  organic 


116 


AQUEOUS   CAUSES. 


world.  The  lacustrine  deposits  can  contain  only  fresh-water  species 
of  animals,  marine  deposits  only  marine  animals,  while  deltas  may 
contain  the  remains  of  marine  life  mingled  with  those  which  have 


Fi; 


been  washed  down  by  rivers.  The  remains  of  birds,  insects,  and 
terrestrial  animals,  may  occasionally  occur,  in  every  kind  of  deposit. 
Sediment  deposited  in  deep  water  will  never  contain  fossils  in 
abundance,  the  deep  parts  of  the  ocean  being  almost  wholly  desti- 
tute of  animal  or  vegetable  life.  It  is  only  in  water  of  a  few 
fathoms  that  the  greater  number  of  species  and  of  individuals 
occur.  In  all  these  particulars  the  deposits  now  forming  sustain  a 
close  resemblance  to  the  older  formations. 

There  are  certain  formations,  as  that  of  the  coal,  which  required 
conditions  for  their  formation  different  from  those  of  ordinary 
sedimentary  deposits.  Coal  consists  of  mineralized  vegetable  mat- 
ter. Its  vegetable  origin  is  proved  by  the  uniform  occurrence  of 
vegetable  fossils  almost  exclusively  in  the  coal  measures.  When 
reduced  to  thin  slices  and  examined  under  a  high  magnifying 
power,  a  structure  very  similar  to  the  ligneous  tissue  of  existing 
coniferae  is  sometimes  found  to  exist.  There  are  probably  vegeta- 


AQUEOUS   CAUSES.  117 

ble  deposits  now  taking  place  not  altogether  unlike  those  which 
produced  the  coal  measures. 

We  know  that  many  rivers — the  Mississippi,  for  example  —  now 
carry  into  the  sea  great  quantities  of  ligneous  matter.  Before  the 
country  was  inhabited  by  man,  the  quantity  was  undoubtedly  much 
greater  than  it  now  is.  It  floats  for  a  time ;  but  the  ligneous  tissue 
itself  is  heavier  than  water,  and  as  soon  as  the  air  is  excluded  from 
the  pores,  and  they  are  filled  with  water,  it  will  sink.  The  woody 
and  earthy  matters  are  swept  into  the  sea  together ;  but,  as  they 
sink  under  different  circumstances,  they  will  be  deposited  separately. 
Thus  wood  may  continue  to  accumulate  in  particular  places  in 
the  sea  for  long  periods,  with  but  little  intermixture  of  earthy 
substances. 

It  is,  however,  to  be  expected  that,  in  the  progress  of  geological 
changes,  the  places  which  at  one  time  receive  deposits  of  wood  will 
at  another  receive  detrital  matter,  and  thus  the  wood  will  become 
deeply  buried  beneath  sedimentary  strata. 

Wood  thus  situated  will  become  converted  into  coal.  Trees 
which  had  been  covered  to  considerable  depth  with  earth  have 
been  found  near  the  Mississippi  river  changed  to  lignite,  a  sub- 
stance resembling  charcoal.  In  this  case,  the  wood  had  been 
exposed  to  no  greater  heat  than  is  common  to  the  crust  of  the 
earth  at  the  depth  where  it  was  found ;  and  yet  it  had  under- 
gone this  change  since  the  country  has  been  known  to  Europeans, 
as  it  retained  the  marks  of  the  axe  when  it  was  discovered.  It 
has  also  been  found  by  experiment  that  vegetable  matter,  by  long 
submersion  in  water,  passes  into  the  state  of  lignite.  This  is  the 
first  step  in  the  conversion  of  wood  into  mineral  coal. 

When  lignite  is  exposed  to  moderate  heat  and  great  pressure,  it 
loses  the  characters  of  lignite,  and  becomes  mineral  coal.  This 
is  shown  by  facts  observed  in  Germany,  Ireland  and  Iceland,  where 
beds  of  lignite  have  been  overspread  by  basalt.  The  upper  por- 
tions of  the  lignite  arc  changed  to  mineral  coal.  The  lower  por- 
tions, which  the  heat  did  not  reach,  retain  the  characters  of 
lignite. 


118  AQUEOUS    CAUSES. 

Beds  of  vegetable  matter,  with  a  great  thickness  of  rock  depos- 
ited above  them,  would  therefore  be  subject  to  all  the  conditions 
necessary  to  convert  them  into  coal,  namely,  pressure  from  the 
superincumbent  mass,  and  the  heat  which  the  strata  uniformly 
assume  at  great  depths. 

It  is  not  improbable,  therefore,  that  coal-beds  are  now  forming, 
and  that  they  have  been  formed  at  every  geological  period  since 
an  abundant  terrestrial  vegetation  commenced.  Accordingly,  there 
occurs  in  Virginia  an  extensive  coal-field  in  the  oolite  formation. 
Coal-fields  also  occur  in  England,  of  less  extent,  in  the  same  form- 
ation. In  France,  and  other  parts  of  Europe,  there  are  extensive 
beds  of  lignite  in  the  tertiary  formation. 

We  have  therefore  no  difficulty  in  accounting,  in  a  general  way, 
for  the  formations  of  the  carboniferous  period.  The  vegetables 
were  probably  less  woody  than  those  of  the  present  time  of  equal 
size,  and  were  therefore  more  easily  prostrated  and  committed  to 
the  waters.  They  grew  rapidly  in  moist  ground,  and  perhaps  in 
shoal-water,  and  required  an  atmosphere  charged  with  moisture 
and  of  a  high  temperature.  Thus  much  is  inferred  from  the  con- 
ditions most  favorable  for  the  growth  of  recent  species  analogous  to 
the  coal-plants.  These  recen^  species  are  tropical  plants,  and  grow 
in  moist  insular  situations,  —  conditions  which  would  have  existed 
at  the  carboniferous  period,  if  the  present  coal-fields  were  then  an 
archipelago  dotted  with  low  islands. 

Such  being  regarded  as  the  origin  of  the  coal-beds,  the  alterna- 
tions of  the  earthy  and  carbonaceous  strata  may  be  referred,  pro- 
visionally, to  those  great  changes  in  physical  geography  upon  which 
the  other  alternations  of  strata  on  a  large  scale  depend.  But  the 
regularity  with  which  the  coal-seams  and  sandstone  succeed  each 
other  presents  some  difficulties  which,  in  the  present  state  of  knowl- 
edge, we  cannot  satisfactorily  account  for. 

Beds  of  salt  occur,  interstratified  with  other  rocks,  in  nearly  all 
countries.  Still,  it  is  not  a  sedimentary  deposit,  and  its  formation 
must  depend  upon  peculiar  circumstances.  In  New  York,  saline, 
together  with  earthy  matter,  constitutes  the  Onondaga  limestone, 


AQUEOUS   CAUSES.  119 

one  of  the  formations  of  the  New  York  system.  In  Kentucky,  the 
strata  of  rock-salt  are  in  the  coal  formation ;  in  England,  they  are 
in  the  new  red  sandstone ;  in  Spain,  they  are  in  the  greensand,  and 
m  Poland  they  are  in  tertiary  strata.  The  conditions  of  its  form- 
ation have  therefore  existed  in  connection  with  the  deposition  of 
every  fossiliferous  rock. 

It  has  been  shown  that  the  ocean  is  the  principal  reservoir  of 
the  saline  matters  which  are  taken  up  whenever  water  percolates 
through  rocks.  It  must  happen  not  unfrequently,  in  the  course  of 
submarine  elevations,  that  a  basin  of  sea-water  will  be  cut  off  from 
its  communication  with  the  sea ;  and  from  this  basin  the  evapora- 
tion might  be  more  rapid  than  the  supply  of  water.  The  great 
*alt-lake  of  Utah  is  undoubtedly  a  basin  of  this  kind.  The  Med- 
iterranean Sea  is  another  such  basin,  not  yet  wholly  separated  from 
the  ocean.  The  evaporation  exceeds  the  supply  of  water  from  the 
rivers,  and  a  powerful  stream  is  therefore  continually  thrown  in 
from  the  ocean,  through  the  Strait  of  Gibraltar.  The  waters  of  the 
Mediterranean  are  already  more  highly  charged  with  salt  than 
ordinary  sea-water.  This  sea  may  ultimately  become  a  saturated 
solution,  and  begin  to  deposit  salt.  But  whether  it  does,  or  not,  it 
indicates  the  way  in  which  salt-beds  may  be  formed. 

Y.  Solidification  of  Aqueous  Deposits. 

Sediment  is  generally  deposited  as  a  soft  mud,  but  in  nearly  all 
the  older  formations  it  has  become  solidified.  When  rocks  are 
deposited  from  a  chemical  solution,  they  take  at  once  the  solid 
form.  Such  is  the  case  with  rock-salt  and  with  limestone,  when 
the  material  has  been  held  in  solution.  Solidification  takes  place 
in  nearly  the  same  way  when  water  which  holds  carbonate  of  lime 
or  oxide  of  iron  in  solution  filters  through  beds  of  sand  or  gravel. 
The  substance  held  in  solution  is  deposited  in  the  interstices  till 
they  become  filled,  and  the  whole  is  changed  to  solid  rock. 

Some  rocks  are  composed  of  such  materials  that  they  set,  like 
hydraulic  cement,  when  they  are  deposited.  Other  rocks  become 
solid  simply  by  drying.  Thus  a  deposit  now  forming  in  Lake 
Superior  becomes,  by  drying,  nearly  as  hard  as  granite.  Such  a 


120  AQUEO-GLACIAL   ACTION. 

deposit  will  therefore  become  solid  whenever  it  shall  be  elevated 
above  the  water. 

The  pressure  to  which  all  but  the  upper  layers  are  subjected  is 
probably  sufficient  to  reduce  most  rocks  to  the  solid  state.  Dry 
and  pulverized  clay  is  reduced  by  artificial  pressure,  for  a  moment, 
almost  to  stone.  The  pressure  upon  the  deep-seated  rocks  is  con- 
stant, and  greater  than  any  artificial  pressure  can  be. 

In  addition  to  these  causes,  all  the  older  rocks  have  been  sub- 
jected to  a  high  temperature,  some  of  them  nearly  to  that  of 
fusion.  By  this  means  the  solidification  of  every  kind  of  rock 
would  be  promoted,  and  probably  some  may  have  been  reduced  by 
it  to  the  solid  state,  which  would  otherwise  have  remained  as  an 
incoherent  mass. 


SECTION    V. AQUEO-GLACIAL   ACTION. 

1.  Glaciers. — A  glacier  is  a  mass  of  ice  occupying  the  bed 
of  a  mountain  valley,  having  a  slow  progressive  motion,  and 
reaching  somewhat  lower  in  the  valley  than  the  line  of  constant 
snow.  (Fig.  77.)  The  Glacier  des  Bois,  which  may  be  regarded 
as  a  specimen  of  the  Alpine  glaciers,  covers  an  area  of  about  seven- 
teen square  miles.  In  its  lowest  portion,  when  all  its  branches 
have  become  united  into  one  stream,  it  has  an  average  width  of 
half  a  mile,  and  is  five  miles  long.  It  is  estimated  that  the  gla- 
ciers of  the  Alps  cover  an  area  of  fourteen  hundred  square  miles. 
These  have  been  the  most  carefully  studied,  though  glaciers  are 
found  in  the  valleys  of  various  other  ranges  of  mountains. 

In  the  higher  valleys,  the  snow,  which  falls  at  all  seasons  of  the 
year,  accumulates  in  immense  quantities,  and  the  steep  mountain 
sides  contribute,  by  frequent  avalanches,  to  this  accumulation.  The 
snow,  when  thus  increased,  does  not  become  a  compact,  adhesive 
mass ;  but,  changing  into  particles  of  solid  ice,  it  resembles  sand 
rather  than  snow.  It  is  this  neve  which  constitutes  the  upper  part 
of  every  glacier,  and  which,  in  a  modified  form,  constitutes  the 
lower  part. 


AQUEO-GLACIAL   ACTION.  121 

The  valleys  descend  rapidly  towards  the  base  of  the  mountains ; 
and  this  snow-ice,  having  no  cohesion  between  its  particles,  moves 
slowly  down  the  slope  of  the  valley,  like  a  very  imperfect  liquid. 
After  descending  below  the  line  of  perpetual  snow,  the  surface 
will  melt  during  the  day ;  and  the  water,  sinking  into  the  porous 
mass,  becomes  frozen,  and  converts  the  whole  into  more  or  less 
compact  ice,  yet  never  into  a  rigid  mass.  Influenced  by  its  own 
weight,  and  by  the  pressure  of  the  snow-ice  behind,  it  still  contin- 
ues its  motion,  and  conforms  itself  to  the  shape  and  curves  of  the 
valley  through  which  it  passes.  The  average  movement  per  annum 
may  be  stated  at  about  five  hundred  feet. 

The  temperature  of  the  rocky  bed  of  the  valley  will  be  a  little, 
and  but  a  little,  higher  than  thirty-two  degrees.  There  will  there- 
fore be  but  little  melting  at  the  bed  of  this  river  of  ice.  As  it 
receives  continual  accessions  from  the  atmosphere,  it  will  there- 
fore increase  in  volume  till  it  descends  to  the  level  of  perpetual 
snow.  Below  this  line  the  waste  exceeds  the  addition;  and  as  it 
approaches  the  lower  and  cultivated  portions  of  the  valley,  it  rap- 
idly diminishes,  till  it  finally  loses  the  solid  form,  and  becomes  a 
rivulet.  The  terminus  of  the  glacier  is  determined  principally  by 
the  general  climate  of  the  country.  Any  considerable  variation 
of  climate  will  cause  it  to  recede,  or  descend  lower  down  the  val- 
ley. The  terminus  varies,  however,  somewhat  with  the  seasons, 
being  lower  in  winter  than  in  summer,  though  the  motion  is  much 
less  in  the  cold  season  than  in  the  warm ;  and  it  descends  many 
rods  further  some  seasons  than  it  does  others. 

The  glacier  consists  principally  of  snow,  more  or  less  modified  in 
structure ;  but  it  also  contains  whatever  else  may  have  been  thrown 
upon  its  surface,  or  into  the  snows  by  which  it  is  fed.  Tributary 
glaciers  extend  up  through  all  the  gorges  into  which  the  irregular 
surface  of  the  mountain-top  is  divided.  On  these  rough  peaks 
there  are  always  fragments  of  rock,  varying  in  size  from  fine  sand 
to  masses  weighing  many  tons ;  some  of  them  loosened  when  the 
mountain  was  upheaved,  some  by  subsequent  earthquake  vibra- 
tions, and  others  still  by  tempests,  lightnings,  and  changes  of 
11 


122 


AQUEO-GLACIAL  ACTION. 


temperature.  When  the  snow  has  accumulated  to  a  certain 
extent  on  the  steep  slopes,  it  falls  in  avalanches  into  the  valleys, 
carrying  with  it  loosened  masses  of  rock,  and  often  breaking  off 
large  fragments  from  the  rocky  escarpments  against  which  it 
strikes.  These  avalanches  are  almost  constantly  descending,  and 
hence  a  glacier  always  contains  considerable  earthy  matter  distrib- 
uted through  it. 

Fig.  77. 


The  friction  of  the  glacier,  at  its  edges  and  along  its  bed,  sep- 
arates more  or  less  of  the  rock  over  which  it  moves ;  and  hence 


AQUEO-GLACIAL   ACTION.  123 

there  is  always  a  layer  of  mud  and  pebbles  under  the  glacier,  and 
a  line  of  loose  fragments,  called  a  lateral  moraine,  at  the  sides. 
When  two  glaciers  unite,  the  two  lateral  moraines,  thus  brought 
together,  come  to  the  surface,  forming  a  medial  moraine,  and  show 
the  line  of  junction  sometimes  for  miles. 

The  friction  of  the  glacier  on  the  bed  of  rock,  assisted  by  the 
layer  of  pebbles,  will  wear  down  the  prominent  portions,  and 
everywhere  polish  the  surface.  Fragments  of  rocks  may  be  frozen 
into  the  glacier  at  all  depths.  Those  which  lie  near  the  lower  sur- 
face of  the  glacier  would,  by  slight  melting  of  that  surface,  project 
downward  so  as  to  act  as  a  graver's  tool  on  the  rock  over  which  it 
passes.  Hence,  when  the  extremity  of  the  glacier  has  receded 
beyond  its  ordinary  limit,  the  surface  of  rock  exposed  is  found, 
upon  examination,  to  be  polished,  striated,  and  occasionally  grooved 
an  inch  or  two  deep. 

Since  the  waste  is  almost  wholly  superficial,  earthy  matter,  which 
was  at  first  concealed  in  the  mass  of  the  glacier,  is  continually  com- 
ing to  view,  as  the  surface  melts  and  runs  oft7.  Thus,  none  of  the 
freight  of  the  glacier  is  left  along  its  course,  but  all  is  carried  to 
its  terminus  and  discharged  there.  Hence,  at  the  lower  extremity 
of  the  glacier  there  is  always  an  embankment  of  earth,  pebbles, 
and  boulders.  If  the  glacier  recedes  a  few  yards  at  one  season  of 
the  year,  and  leaves  its  earthy  fragments  scattered  over  this  surface, 
•  they  will  be  pushed  forward  into  a  ridge,  as  the  glacier  again 
advances.  This  ridge  is  called  a  terminal  moraine,  and  consists 
wholly  of  substances  which  have  been  separated  from  the  mountain 
mass,  often  at  the  highest  beginnings  of  the  glacier.  At  the  ter- 
minus of  all  the  Alpine  glaciers,  there  is  a  series  of  these  moraines 
(a  a  a,  Fig.  77)  marking  the  successive  limits  of  the  glacier  in 
former  times. 

There  is  a  ridge  of  boulders  on  the  north  side  of  the  Swiss  val- 
ley, near  the  base  of  the  Jura  Mountains,  resembling  a  terminal 
moraine.  These  boulders  consist  of  several  groups,  distinguished 
by  peculiarities  of  structure  and  composition ;  and  each  group  lies 
opposite  to  the  particular  Alpine  valley  which  now  furnishes  the 


124  AQUEO-GLACIAL   ACTION. 

same  kind  of  fragments.  It  has  been  thought  that,  at  a  former 
period  of  more  severe  climate,  the  Swiss  valley  was  filled  in  part 
with  ice,  and  that  the  present  glaciers  extended  across  it  to  the 
Jura  Mountains. 

It  is  found  that  the  polished  and  striated  surfaces  of  the  rocks 
in  the  Alpine  valleys  are  precisely  like  the  surface  of  the  rock, 
which  has  not  been  exposed  to  atmospheric  influences,  in  the 
north  of  Europe  and  America.  It  has  been  proposed  to  extend 
the  glacier  theory,  and'  account  for  these  phenomena  by  supposing 
that  the  north  polar  regions  were,  at  the  ice  period,  capped  with 
a  glacier-mass,  extending  as  far  south  as  the  drift  phenomena 
appear. 

It  is  not  to  be  doubted  that  the  phenomena  of  polished  surfaces 
and  transported  materials  in  the  immediate  vicinity  of  the  Alps, 
and  near  other  high  mountains,  are  correctly  referred  to  glacial 
action.  This  theory  has  therefore  solved,  in  part,  one  of  the  most 
difficult  problems  in  geology;  but  there  is  great  difficulty  in 
extending  it  so  as  to  account  for  the  drift  phenomena  in  general. 
If  the  motion  depends  upon  gravitation  only,  the  origin  must  have 
a  much  greater  elevation  than  the  terminus,  which  would  not  be 
the  case  in  the  great  glacier  supposed  to  extend  southward  from 
the  Arctic  regions.  Elevation  of  temperature,  it  has  been  thought, 
might  account  for  the  movement  of  the  mass  southward. 

2.  Icebergs.  —  In  very  high  latitudes,  the  ice,  which  makes  out 
from  the  land  into  the  sea  during  the  cold  season,  suffers  but  little 
waste  at  any  time.  This  sheet  of  ice  continues  to  increase  in 
breadth  and  thickness,  by  congelation,  from  year  to  year.  The 
spray  and  the  snows  of  each  succeeding  year  will  also  add  to  the 
mass.  It  thus  accumulates  to  the  height  of  several  hundred 
yards.  It  will  also  reach  down  a  good  many  feet  below  the  surface 
of  the  sea,  and  will  extend  back  on  the  land,  or  lie  heaped  up 
against  a  precipitous  escarpment,  and  firmly  frozen  to  it. 

After  a  certain  amount  of  extension  over  the  sea,  the  accumu- 
lated weight  of  the  ice  and  snow  would  tend  to  depress  it,  and 
break  it  loose  from  the  shore.  The  waves  would  tend  to  the  same 


AQUEOGLACIAL   ACTION.  125 

result,  and  would  act  at  greater  mechanical  advantage,  as  its  exten- 
sion from  the  shore  becomes  greater.  Hence,  it  would  ultimately 
become  separated  from  the  shore,  and  float  in  the  water. 

At  its  commencement,  the  earth,  pebbles  and  rocks,  which  may 
lie  along  the  shore,  and  as  far  down  into  the  sea  as  the  congela- 
tion extends,  are  frozen  into  it.  In  many  situations  its  mass  would 
be  increased  by  avalanches  while  it  remained  attached  to  the  land, 
and  these  would  supply  also  masses  of  earth  and  rocks,  as  they  do  to 
glaciers.  When  it  becomes  loosened  from  the  shore,  it  will  break 
off,  and  carry  with  it  some  of  the  earthy  portions  of  the  coast,  or 
the  less  firmly  fixed  masses  of  rock  from  the  escarpment  against 
which  it  formed.  Thus  every  iceberg  becomes  freighted,  more 
or  less,  with  earth  and  rocks.  This  has  almost  uniformly  been 
found  to  be  the  case,  when  they  have  been  landed  upon  by  ships' 
crews  and  examined. 

"We  have  seen  that  the  general  tendency  of  the  waters  of  the 
ocean,  and  of  the  lower  stratum  of  the  atmosphere,  is  to  a  motion 
from  the  poles  towards  the  equator.  However  irregular,  there- 
fore, the  course  of  an  iceberg  may  be,  its  general  movement,  influ- 
enced both  by  the  prevailing  winds  and  by  ocean  currents,  will  be 
towards  the  equator. 

These  floating  ice-mountains  (Fig.  78)  are  formed  in  great  num- 
Fig.  78. 


bers,  and  of  vast  size.     The  relative  specific  gravity  of  ice  and 
water  are  such  that  nine  cubic  feet  of  ice,  below  the  surface  of 
11* 


126  AQUEO-GLACIAL   ACTION. 

water,  will  support  one  cubic  foot  above  it.  As  icebergs  are  often 
one  or  two  hundred  feet  high,  their  vertical  depth  must  be  a  thou- 
sand feet  at  least ;  and  their  area  is  equal  to  a  square  mile,  and 
sometimes  it  is  much  greater.  In  1840,  the  United  States  Explor- 
ing Expedition,  in  the  extreme  southern  ocean,  coasted  for  eighty 
miles  along  a  single  iceberg.  They  are  never  absent  from  the 
polar  seas  ;  and  at  certain  seasons  they  are  so  abundant  along  the 
usual  course  of  vessels  from  New  York  to  Liverpool,  as  greatly 
to  obstruct  and  endanger  navigation. 

An  iceberg  may  continue  for  some  time  to  increase  in  size,  while 
floating  in  the  polar  seas,  but  will  at  length  reach  a  latitude  where 
the  waste  will  exceed  the  additions,  in  consequence  of  the  tempera- 
ture both  of  the  air  and  of  the  water.  It  will,  therefore,  drop 
gradually  the  earthy  matters  which  it  contains,  upon  the  bed  of 
the  ocean. 

It  is  not  improbable  that  icebergs  may  often  reach  down  so  far 
as  to  strike  the  highest  points  of  the  bed  of  the  sea.  The  ice 
would  be  lifted,  and  glide  over  the  elevation,  without  suffering  any 
perceptible  deviation  from  its  general  course.  It  would  thus  affect 
the  surface  of  rocks  exactly  like  a  glacier.  If,  however,  the  ice- 
berg becomes  permanently  stranded,  and  melts  in  one  place,  its 
earthy  matters  will  be  thrown  down  upon  the  elevation  which  first 
arrested  it. 

If  the  bed  of  the  sea,  between  the  fortieth  and  sixtieth  degrees 
of  latitude,  could  be  exposed  for  examination,  the  rocky  surface 
would  be  found  to  be  polished  and  striated  by  the  icebergs  which 
have  passed  over  it,  and  the  whole  surface  would  be  strewed  with 
boulders  and  drifted  materials  brought  from  Arctic  and  Antarctic 
lands.  Sometimes  it  would  be  accumulated  in  heaps,  and  some- 
times spread  nearly  over  the  surface. 

We  have  seen  that  very  recently,  probably  about  the  close  of  the 
tertiary  period,  the  portion  of  Europe  and  America  over  which  the 
northern  drift  is  found,  has  been  depressed  several  hundred  feet. 
It  may  be  presumed  that  at  that  time  icebergs  floated  over  it,  pol- 
ished the  surface  of  the  rocks,  and  distributed  the  boulders  and 
other  drift  which  is  now  found  upon  it. 


IGNEOUS   CAUSES.  127 


SECTION    TI. IGNEOUS   CAUSES. 


I.  Of  the  Temperature  of  the  Mass  of  the  Earth.  —  Heat  has 
been  the  most  efficient  agent  in  determining  and  modifying  the 
structure  of  the  earth ;  and,  in  order  that  the  explanations  of  the 
phenomena  referable  to  this  cause  may  be  intelligible,  some  idea 
must  be  formed  of  the  actual  present  condition  of  the  mass  of  the 
earth  with  respect  to  heat. 

At  any  point  of  the  surface  there  are  variations  of  temperature, 
depending  on  external  causes.  But  these  variations  are  found  to 
extend  only  a  little  way  below  the  surface,  —  never  more  than  a 
hundred  feet.  At  greater  depths,  it  is  found  that  the  temperature 
invariably  increases  with  the  depth.  Deep  mines  have  always  a 
temperature  above  the  mean  annual  temperature  at  the  surface. 
The  water  obtained  by  deep  boring  is  always  tepid  when  it  comes 
to  the  surface.  The  thermal  springs,  so  abundant  in  this  country 
and  in  Europe,  are  so  situated  as  to  justify  the  impression  that 
their  waters  come  from  great  depths.  To  make  these  general 
observations  of  any  value,  we  must  determine  the  law  by  which 
the  temperature  increases.  The  result  of  all  the  observations  yet 
made,  in  mines  and  upon  wells  and  springs,  is  that,  below  the  first 
hundred  feet,  the  temperature  increases  by  one  degree  of  Fahren- 
heit's scale  for  every  forty-five  feet. 

Regarding  this  law  of  increment  as  applicable  to  all  depths,  at 
ten  miles  below  the  surface  we  should  have  a  temperature  above 
that  produced  by  the  combustion  of  wood;  and  at  twenty-five 
miles,  a  temperature  of  three  thousand  degrees,  by  which  nearly 
all  mineral  substances  would  be  reduced  to  a  state  of  fusion. 

The  general  conclusion  of  a  temperature  sufficient  to  melt  the 
mineral  substances  of  which  rocks  are  composed,  at  no  considerable 
distance  below  the  surface,  is  confirmed  by  the  fact  that  portions  of 
the  interior  of  the  earth  —  at  least,  at  the  volcanic  centres  —  are  in 
a  melted  state.  The  intimate  connection  between  some  volcanoes 
situated  a  hundred  miles  or  more  apart,  so  that  they  are  alternately 
in  a  state  of  activity  and  rest,  indicates  that  these  centres  are  con- 


128  IGNEOUS    CAUSES. 

nected,  —  that  subterranean  melted  lava  extends  from  one  to  the 
other,  so  that  when  one  is  active,  the  elastic  force  is  relieved  at 
the  other.  These  deep-seated  lakes  of  lava  must  therefore  under- 
lie large  areas. 

We  are  justified,  then,  in  concluding  that  the  mass  of  the  earth, 
with  the  exception  of  a  comparatively  thin  superficial  layer,  has  a 
very  high  temperature. 

By  way  of  accounting  for  this  temperature,  it  is  -now  generally 
assumed  that  the  earth  was  originally  in  a  state  of  fusion ;  that  it 
was  a  mass  of  liquid  lava  (if,  indeed,  it  had  not  a  temperature  suf- 
ficient to  reduce  it  to  the  aeriform  state).  Starting  with  this 
assumption,  there  must  necessarily  be  a  gradual  reduction  of  tem- 
perature by  radiation,  and  a  time  must  arrive  when  the  surface 
would  be  crusted  over  with  solidified  lava  ;  and  this  crust  would 
increase  in  thickness  as  the  cooling  advanced,  the  interior  still 
retaining  its  heat  and  liquidity.  The  present  condition  of  the  crust 
of  the  earth,  its  forrrr,  that  of  an  oblate  spheroid,  with  the  exact 
difference  of  the  equatorial  and  polar  diameters  which  is  found  to 
exist,  as  well  as  the  phenomena  of  volcanic  eruptions,  will  all 
admit  of  explanation  on  this  hypothesis. 

It  has,  however,  been  rejected  by  some  ;  and,  to  account  for  the 
heat  of  the  interior  of  the  earth,  it  is  suggested  that,  if  the  bases 
of  the  earths  and  alkalies,  particularly  potassium,  sodium  and  cal- 
cium, exist  in  their  metallic  state  beneath  the  surface,  the  rapid 
oxidation  of  them  by  the  access  of  water  would  generate  heat  of 
sufficient  intensity  to  melt  the  oxidized  materials,  and  thus  account 
for  the  phenomena  attributable  to  heat. 

Either  of  these  hypotheses  may  be  adopted ;  but  it  is  not  neces- 
sary to  account  at  all  for  the  existence  of  this  temperature.  The 
fact  is  susceptible  of  proof;  and,  though  we  may  not  be  able  to 
frame  any  hypothesis  to  account  for  its  existence,  we  may  yet 
employ  the  fact  in  the  explanation  of  other  phenomena. 

II.   The  Action  of  Internal  Heat  in  producing  Volcanoes. 

The  phenomena  of  volcanoes  and  earthquakes  are  evidently  pro- 
duced by  some  force  operating  from  below.  The  effect  of  heat 


IGNEOUS   CAUSES.  129 

alone  would  be  to  reduce  the  rock  to  a  liquid  state.  There  is  no 
reason  to  suppose  that  it  is  ever  sufficient  to  reduce  them  to  the 
aeriform  state.  The  elastic  force  must  therefore  depend  upon 
some  other  substance  associated  with  the  lava,  and  this  substance 
is  water. 

This  will  be  shown  by  an  examination  of  lavas.  At  the  time 
of  their  ejection,  they  are  in  a  fluid  or  semi-fluid  state;  but  it  is  not 
a  complete  fusion.  Even  the  most  fluid  lavas  contain  particles  of 
minerals  in  a  solid  state.  The  liquidity  depends  upon  the  fusion 
of  the  more  fusible  portions,  and  upon  the  steam  of  water  at  a 
high  temperature,  which  fills  the  interstices  between  the  solid  par- 
ticles. The  porous  character  of  cooled  lavas  is  produced  by  the 
steam  which  filled  the  cavities  previous  to  solidification.  Steam 
always  escapes  from  the  surface  of  a  lava  current  while  it  is  cool- 
ing, and  it  is  always  discharged  in  immense  volumes  from  the 
orifice  of  eruption,  in  connection  with  the  lava,  and  especially  at 
the  close  of  an  eruption. 

The  geographical  position  of  volcanoes,  also,  leads  to  the  con- 
clusion that  water  is  essential  to  their  activity.  There  are  five 
principal  lines  of  volcanic  activity.  One,  commencing  at  the 
southern  extremity  of  South  America,  extends  northward  along 
the  Andes  and  Cordilleras  to  California  or  Oregon.  The  second 
has  a  north-east  and  south-west  direction,  from  the  Aleutian 
Islands  through  the  Kurule,  Japanese,  and  Philippine  islands, 
till  it  meets  the  third  line,  lying  in  a  nearly  east  and  west  direction, 
embracing  Sumatra,  Java,  and  most  of  the  Pacific  volcanic  islands. 
A  fourth  band  commences  in  the  Grecian  islands,  and  extends 
westward  so  as  to  include  the  volcanoes  of  Italy  and  the  adjacent 
islands,  and  the  Azores.  The  fifth  band  embraces  the  volcanic 
islands  of  the  West  Indies,  crosses  Mexico  in  about  the  latitude 
of  the  city  of  Mexico,  and  extends  into  the  Pacific.  There  are 
also  some  isolated  centres  of  volcanic  activity,  such  as  Iceland. 
These  volcanic  bands  embrace  about  three  hundred  volcanoes.  It 
will  be  seen  that  they  must  nearly  all  be  in  close  proximity  to  the 
ocean,  or  to  large  seas.  About  two-thirds  of  them  are  on  islands. 


130  IGNEOUS    CAUSES. 

Moreover,  the  volcanic  vents  which  are  wholly  submarine  are 
probably  very  numerous. 

This  circumstance  of  the  position  of  volcanoes  establishes  a  pre- 
sumption that  they  cannot  exist  at  a  distance  from  some  large 
body  of  water ;  and,  taking  it  in  connection  with  the  constant 
presence  of  aqueous  vapor  in  lava,  we  are  justified  in  the  conclu- 
sion that  the  presence  of  water  is  an  essential  condition  of  volcanic 
'activity. 

Knowing  that  heat  and  water  exist  at  the  volcanic  centres,  it  is 
not  difficult  to  form  an  idea  of  their  mode  of  operation.  The 
water,  diffused  through  the  interstices  of  the  lava,  and  subjected  to 
a  temperature  sufficient  to  melt  the  lava,  would  possess  an  elastic 
power,  which,  though  never  computed,  we  may  well  suppose  capa- 
ble of  overcoming  any  resistance  which  the  crust  of  the  earth 
might  present.  The  repressing  force  will  be  the  tenacity  and 
weight  of  the  superincumbent  strata.  Whenever  the  elasticity  is 
superior  to  this  repressing  force,  it  will  manifest  itself  in  the  frac- 
ture of  the  strata,  and  often  in  the  ejection  or  lava  to  the  surface. 

This  fracturing  of  the  strata,  produced  by  an  uplifting  subter- 
ranean force,  is  believed  to  be  the  cause  of  the  noise  and  the  vibra- 
tory motion  which  are  the  chief  phenomena  of  earthquakes.  The 
elastic  force  may  raise  lava  to  the  surface,  and  thus  the  fracture 
would  become  a  volcano.  But  the  force  may  expend  itself  by  the 
discharge  of  vapor  into  the  fissure,  or  by  merely  filling  it  with 
lava.  In  either  case,  the  only  evidence  of  the  existence  of  the 
volcanic  force  would  be  the  noise  and  the  wave-like  motion  expe- 
rienced at  the  surface.  The  cause  of  the  volcano  and  earthquake 
is  therefore  the  same,  though  the  phenomena  which  characterize 
them  are  different. 

When  the  strata  are  is  thus  fractured,  lava  may  for  a  time  be 
discharged  along  the  whole  line.  By  the  cooling  of  lava  in  the 
fracture,  it  would  become  partially  reunited.  Still,  this  would 
be  the  line  of  least  resistance.  It  would  therefore  be  again  burst 
through  in  certain  places,  which  would  long  continue  to  be  orifices 


IGNEOUS  CAUSES.  131 

of  discharge,  and  thus  the  original  fracture  would  determine  a  line 
of  volcanic  activity. 

The  repressing  force  may  become  greater  at  an  orifice  of  erup- 
tion than  at  some  other  point,  either  by  the  great  accumulation  of 
ejected  materials  around  the  opening,  or  by  the  dormancy  of  the 
volcano  long  enough  for  the  complete  solidification  of  the  lava 
with  which  the  channel  was  filled.  The  least  resistance  may  then 
be  far  from  any  previous  vent,  when  a  new  orifice  of  discharge 
will  be  opened,  and  a  new  volcano  make  its  appearance.  It 
seems  probable,  also,  that  volcanoes  may  become  extinct  by  the 
reduction  of  temperature  at  the  volcanic  centre,  and  that  new  vol- 
canic centres  may  be  formed ;  but  the  cause  of  this  change  of  tem- 
perature is  not  yet  well  understood.  New  volcanoes  have  broken 
out  in  the  sea,  near  Iceland,  in  several  instances ;  others  in  the 
volcanic  line  east  of  Asia.  Graham  Island,  situated  between  Sic- 
ily and  Africa,  was  formed  by  an  eruption  which  broke  out  in  the 
bed  of  the  sea  where  the  soundings  were  more  than  one  hundred 
fathoms.  The  island  was  at  one  time  two  hundred  feet  'above  the 
sea,  and  three  miles  in  circumference.  It  was,  however,  gradually 
destroyed  by  the  action  of  the  waves,  and  now  remains  a  danger- 
ous reef,  covered  by  less  than  two  fathoms  water.  The  volcano 
of  Jorullo,  in  Mexico,  was  formed  in  this  way.  Previous  to  the 
formation  of  the  mountain,  the  region  where  it  now  is  was  a  culti- 
vated table-land.  During  the  year  1759  volcanic  action  com- 
menced and  continued,  until,  at  the  expiration  of  twelve  months,  a 
cone  had  been  formed  having  an  elevation  of  sixteen  hundred  feet 
above  the  adjacent  plain. 

An  orifice  of  eruption  is  at  first  but  little  elevated  above  the 
general  surface;  but,  by  the  accumulation  of  ejected  matter,  a  cone 
is  at  length  formed  around  the  vent.  The  upper  portion  of  a  cone 
always  consists  of  these  materials,  but  there  may  also  be  in  pro- 
gress a  general  elevation  of  that  part  of  the  earth's  crust,  and  the 
cone  will  partake  of  that  general  elevation.  The  cones  of  the 
Andes  owe  their  height,  in  a  great  measure,  to  a  general  movement 


132 


IGNEOUS    CAUSES. 


of  elevation ;  those  of  .ZEtna  and  Vesuvius,  in  a  greater  degree,  to 
accumulation  of  ejected  matter. 

In  either  way,  the  height  may  become  so  great  that  the  force 
necessary  to  raise  a  column  of  lava  to  the  top  would  be  greater 
than  the  sides  of  the  cone,  weakened  as  they  always  are  by  frac- 
tures in  all  directions,  can  sustain.  Hence,  the  highest  craters  of 
^tna  and  South  America  have  long  been  closed,  and  the  lava 
escapes  through  fissures  at  a  lower  level,  and  lateral  cones  are 
produced. 

The  form  which  the  materials  have,  when  ejected  from  volca- 
noes, depends  mainly  upon  the  degree  of  liquidity  of  the  lavas  at 
the  volcanic  foci.  If  the  liquidity  is  very  perfect,  the  aqueous 
vapor  will  readily  rise  through  the  lava.  The  steam  thus  sepa- 
rated will  drive  before  it  whatever  rocks,  or  previous  lavas,  may 
obstruct  it.  In  their  progress  they  would  be  reduced  to  sand  and 
powder,  and  ejected  as  volcanic  cinders.  (Fig.  79.)  If  the  lava 

Fig.  79. 


possess  considerable  viscidity,  the  aqueous  vapor  will  separate  with 
more  difficulty,  and  the  lava  and  vapor  will  ascend  the  channel 
together.  Large  bubbles  of  vapor  will,  however,  collect  with 
more  or  less  of  frequency ;  and,  as  they  rise  through  the  lava, 
will  drive  forward  a  portion  of  it,  and  cause  the  overflow  to  take 
place  by  pulsations.  As  the  bubbles  reach  the  surface,  their  burst- 


IGNEOUS   CAUSES.  133 

ing  causes  the  loud  reports,  which  are  compared  to  the  discharge 
of  heavy  artillery.  With  each  explosion  some  of  the  lava  will  be 
projected  violently  into  the  air,  and,  cooling,  will  fall  to  the  sur- 
face as  scoriez,  —  or,  if  the  lava  be  highly  vitreous,  it  will  be 
drawn  out  into  fibres,  and  descend  as  volcanic  glass. 

III.  Geological  Phenomena  referable  to  Volcanic  Action. 

Volcanic  agency  has  probably  never  been  less  than  it  is  now,  and 
we  ought  therefore  to  find  its  effects  very  general  and  important. 

1.  The  most  obvious  of  these  effects  are  the  fractures  with 
which  the  crust  of  the  earth  is  everywhere  intersected.  The 
uplifting  force  upon  which  all  volcanic  phenomena  depend  would 
necessarily  fracture  the  crust,  and  the  wave-like  motion  resulting 
from  the  fracture  would  cause  numerous  secondary  fractures,  hav- 
ing a  parallel  direction.  They  are  often  of  such  extent,  during 
earthquakes,  as  to  endanger  life.  During  the  great  earthquake  at 
Lisbon,  in  1755,  a  fracture  opened  of  sufficient  width  to  swallow 
up  the  quay,  and  several  thousands  of  persons  who  had  fled  there 
for  safety.  The  chasm  remained  permanently  open  to  the  depth 
of  six  hundred  feet.  The  earthquakes  with  which  the  valley  of 
the  Mississippi  was  visited  in  1811  so  often  fissured  the  surface, 
that  the  inhabitants  protected  themselves  by  clinging  to  the  trunks 
of  trees,  which  they  felled  transversely  to  the  direction  of  the 
fissures. 

The  first  fracture  which  is  produced  by  the  upheaving  force  will 
open  upwards,  and  scarcely  reach  down  to  the  seat  of  the  force. 
But  there  will  be  other  parallel  fractures,  dependent  upon  the  first, 
and  opening  downward.  Thus,  the  primary  fracture  at  a  (Fig.  80) 
will  be  at  once  followed  by  the  fracture  pig  80. 

6,  opening  toward  the  lava,  which  will 
be  injected  into  it,  and  which,  on  cool- 
ing, will  form  a  dike.  Their  forma- 
tion is  mostly  concealed  from  observa- 
tion, but  not  always.  During  the  eruption  of  ^Etna,  in  1669, 
numerous  fissures  opened,  one  of  which  was  six  feet  wide  and 
twelve  miles  in  length ;  and  the  light  emitted  from  it  indicated 
12 


134  IGNEOUS   CAUSES. 

that  it  was  filled  with  lava  to  near  the  surface.     The  process  was 
as  perfectly  seen  as  from  the  nature  of  the  case  it  could  be. 

2.  The  conversion  of  the  lower  sedimentary  strata  into  meta- 
morphic  rocks  has  been  effected  by  volcanic  heat.  The  material 
of  which  dikes  consist  has  been  injected  in  a  highly-heated  state  ; 
and,  by  observing  the  effect  which  they  have  had  upon  the  adjacent 
rocks,  we  may  judge  of  the  effect  which  subterranean  heat  must 
have  upon  the  lower  mechanical  strata.  Wherever  the  dikes  are 
of  considerable  thickness,  they  have  converted  the  adjacent  shales 
into  primary  slate,  the  sandstones  into  quartz  rock,  and  the  dark 
and  friable  limestones  into  granular  marble,  and  destroyed  the 
organic  impressions.  In  the  southern  extremity  of  Norway  there 
is  a  district  in  which  granite  protrudes  in  a  large  mass  through 
fossiliferous  strata.  These  strata  are  invariably  altered  to  a  dis- 
tance of  from  fifty  to  four  hundred  yards  from  the  granite.  The 
shales  have  become  flinty,  and  resemble  jasper;  and  near  the 
granite  they  contain  hornblende.  The  siliceous  matter  of  the 
shales  has  become  quartz  rock,  which  sometimes  contains  horn- 
blende and  mica,  and  therefore  constitutes  a  kind  of  granite.  The 
limestone,  which  at  points  remote  from  the  injected  rock  is  an 
earthy,  blue,  coralline  limestone,  has  become  a  white,  granular 
marble,  near  the  granite,  and  the  corals  are  obliterated.  The 
altered  shales  and  limestones  in  many  places  contain  garnets,  ores 

Fig.  81. 


of  iron,  lead,  &c.     The  annexed  (Fig.  81)  is  a  plan  of  this  granite 
and  altered  rock. 


IGNEOUS   CAUSES.  135 

One  of  the  most  instructive  examples  of  metamorphic  action  in 
this  country  is  found  in  the  White  Mountains  of  New  Hampshire. 
These  mountains  have,  till  recently,  been  thought  to  consist  prin- 
cipally of  granite ;  but  it  is  now  ascertained  that  this  supposed 
granite  is  an  altered  rock  of  the  silurian  period.  It  is  represented 
as  "  intersected  by  veins  of  felspathic  granite ;  and  the  general 
mass  is  itself  in  many  parts  converted  into  a  near  approximation 
to  a  Jbinary  granite,  composed  of  distinctly  developed  quartz  and 
white  felspar,  with  a  few  sparsely  scattered  specks  of  mica.  In 
its  weathered  surfaces  it  wears  a  close  resemblance  to  some  fine- 
grained granites;  but,  upon  inspecting  a  fresh  fracture  with  a 
magnifier,  we  instantly  perceive  many  rounded  grains  of  quartzose 
sand,  and  the  felspar  is  imperfectly  formed,  though  the  mica  has 
more  nearly  reached  the  condition  which  it  has  in  granite.  In 
some  of  the  coarse  varieties  of  this  white  rock,  small  rounded  peb- 
bles of  quartz  are  to  be  seen,  giving  unequivocal  evidence,  even  to 
the  naked  eye,  of  its  being  an  altered  sandstone.  We  feel  no  hes- 
itation in  deciding  it  to  have  been  a  silico-argillaceous  white  sand- 
stone, now  almost  granitized  by  extensive  metamorphic  action." 

Similar  illustrations,  on  a  small  scale,  may  be  seen  in  every 
country  where  the  strata  have  been  cut  through  by  intrusive  dikes. 
Sir  James  Hall  has  shown  the  same  by  actual  experiment.  He 
exposed  pulverized  chalk  to  heat  sufficient  to  melt  it,  and  under 
sufficient  pressure  to  prevent  the  escape  of  the  carbonic  acid. 
After  cooling,  the  chalk  was  found  to  have  taken  the  form  of  crys- 
tallized limestone.  But  instances  enough  have  been  given  to  show 
what  changes  should  be  looked  for  wherever  the  sedimentary  rocks 
have  been  exposed  to  a  high  temperature. 

The  lower  strata  must  have  been  exposed,  for  long  periods  of 
time,  to  such  a  temperature.  We  do  not  know  at  what  depth 
below  the  surface  of  the  earth  the  rocks  become  liquid  ;  but  above 
"the  line  of  actual  fusion  there  must  be  a  mass  of  rock  not  melted, 
yet  scarcely  retaining  the  solid  form.  For  a  great  thickness,  per- 
haps for  several  miles,  it  would  be  in  a  more  or  less  yielding  state. 
As  there  is  not  actual  fusion,  the  stratification  is  not  destroyed, 


136  IGNEOUS   CAUSES. 

but  such  a  degree  of  mobility  among  the  particles  exists,  that  some 
degree  of  crystallization  takes  place,  and  the  elastic  forces  below 
easily  bend,  throw  into  folds,  compress,  and  in  every  way  contort 
these  strata.  At  the  same  time,  any  organic  matters  which  they 
may  contain  are  decomposed,  and  the  impressions  of  them  are 
obliterated.  And  such  is  the  condition  in  which  the  metamorphic 
strata  are  actually  found. 

3.  Denudation  is,  in  a  great  measure,  dependent  on  volcanic 
action.  It  results  from  the  billowy  motion  peculiar  to  the  earth- 
quake. This  is  not  simply  a  violent  horizontal  motion,  but  an 
equally  violent  vertical  one.  It  is  a  series  of  waves,  —  a  succes- 
sion of  alternate  elevations  and  depressions  of  the  solid  crust. 
The  height  of  these  waves  can  only  be  judged  of  by  their  effects ; 
but  it  is  difficult  to  account  for  some  of  these  effects,  without  sup- 
posing the  waves  to  have  been  several  yards  in  height,  and  their 
velocity,  in  the  few  instances  in  which  the  time  has  been  accu- 
rately determined,  was  twenty  miles  a  minute. 

That  such  earthquake  waves  actually  exist  there  can  be  no  doubt. 
During  the  earthquake  in  Calabria,  in  1783,  the  flagstones  in 
many  of  the  towns  were  lifted  from  their  places  and  thrown  down 
inverted,  and  trees  bent  so  that  their  tops  touched  the  ground. 
During  the  great  earthquake  in  Chili,  in  1835,  the  walls  of  houses, 
which  were  parallel  to  the  line  of  oscillation,  were  thrown  down, 
while  those  that  were  at  right  angles  to  it,  though  greatly  frac- 
tured, were  often  left  standing.  Wherever  careful  observations 
have  been  made,  during  and  after  severe  earthquakes,  analo- 
gous facts  have  been  noticed.  Persons  are  generally  affected  with 
sea-sickness.  The  sea  is  violently  agitated.  It  often  retires  to 
an  unusual  distance,  and  then  returns  upon  the  shore  with  most 
destructive  waves.  Incredible,  therefore,  as  it  may  seem,  that  the 
solid  crust  of  the  earth  should  be  thrown  into  such  wave-like  undu- 
lations, the  fact  is  well  established. 

With  a  velocity  of  twenty  miles  an  hour,  the  successive  waves 
may  be  some  miles  apart,  and  yet  be  sufficient  to  account  for  all 
the  phenomena.  It  is  evident,  therefore,  that  the  curvature  of  the 


IGNEOUS  CAUSES.  137 

wave  will  be  very  slight,  and  yet  enough  to  break  into  fragments 
all  the  rocks  thus  curved.  During  the  earthquake  in  Chili, 
before  referred  to,  "  the  ground  was  fissured,  in  many  parts,  in 
north  and  south  lines.  Some  of  the  fissures  near  the  cliffs  were  a 
yard  wide.  Many  enormous  masses  had  fallen  on  the  beach.  The 
effect  of  the  vibrations  on  the  hard  primary  slates  was  still  more 
curious.  The  superficial  parts  of  some  narrow  ridges  were  as 
completely  shivered  as  if  they  had  been  blasted  by  gunpowder." 
Similar  phenomena  seem  everywhere  to  be  exhibited  by  earth- 
quakes. 

It  may  be  presumed  that  almost  all  parts  of  the  earth  have,  at 
different  periods,  been  subject  to  these  earthquake  waves.  Accord- 
ingly, we  find  that  the  crust  of  the  earth  is  nowhere  in  an  entire 
state,  but  is  divided  by  irregular  lines  into  comparatively  small 
fragments.  By  this  means,  the  deep  fissures  produced  by  fractures 
opening  upwards  would  be  filled  with  fragments  of  rock  shattered 
from  the  uplifted  edges.  In  this  way  the  boulder  masses  were 
originally  loosened  from  their  parent  beds,  and  exposed  to  the 
action  of  ice,  or  any  other  transporting  agencies.  In  the  same 
way  the  rocky  bed  of  the  ocean  is,  to  a  considerable  depth,  reduced 
to  a  disintegrated  mass.  In  this  condition  it  will  be  rapidly 
removed  by  marine  currents,  more  or  less  broken,  worn  and  com- 
minuted, by  the  movement,  and  deposited  elsewhere.  The  materi- 
als have  thus  been  furnished  for  a  very  large  proportion  of  the 
sedimentary  rocks,  and  especially  of  those  which  are  composed  of 
distinct  fragments  of  other  rocks.  By  this  means,  also,  wherever 
the  rock  formations  come  to  the  surface,  they  are  so  broken  that 
limestone,  sandstone  or  granite,  suitable  for  architectural  purposes, 
is  seldom  found,  except  at  considerable  depths.  This  fragmentary 
condition  of  the  surface  rock  is  such  as  exposes  it  to  be  acted  upon 
readily  by  any  powerfully  abrading  causes,  or  to  be  more  rapidly 
disintegrated  by  atmospheric  and  aqueous  causes. 

4.  We  have  already  assumed  that  one  principal  division  of  rocks 
—  the  unstratified  —  is  of  igneous  origin.  We  have  the  proof 
of  actual  observation,  that  lavas,  and  the  accompanying  tufas  and 
12* 


138  IGNEOUS    CAUSES. 

grits,  are  volcanic  products.  The  peculiarities  of  these  products, 
in  situation,  structure,  and  form,  and  in  the  imbedded  minerals, 
are  so  great,  that  whenever  we  find  these  peculiarities  in  the  rocks 
of  a  country  not  now  volcanic,  we  still  regard  these  rocks  as  of 
volcanic  origin.  We  thus  have  lavas,  as  well  as  stratified  rocks, 
of  different  ages.  There  has  probably  been  no  time  in  the  earth's 
history  when  they  have  not  been  forming. 

The  trappean  rocks  are  also  of  igneous  origin.  It  is  evident, 
from  their  occurring  in  the  form  of  dikes,  that  they  have  been  in 
a  melted  state.  As  they  rest  upon  rocks  of  a  sedimentary  origin, 
they  must  have  been  thrown  up  by  volcanic  forces.  Yet  they  dif- 
fer from  ordinary  lavas.  They  are  not  vesicular  in  their  struc- 
ture, are  more  crystalline,  and  there  is  in  no  case  evidence  that 
they  have  flowed  from  craters.  If  we  regard  them  as  the  lavas  of 
submarine  volcanoes,  we  shall  have  conditions  which  will  account 
for  all  their  peculiarities.  At  a  certain  depth  the  pressure  of  the 
water  would  be  sufficient  to  prevent  the  formation  and  escape  of 
vapor,  and  therefore  the  lavas  thus  ejected  would  not  be  vesicular. 
As  the  rapid  cooling  of  lavas  depends,  in  a  great  degree,  upon  the 
escape  of  watery  vapor,  submarine  lavas  would  cool  slowly,  in  con- 
sequence of  the  pressure.  The  liquidity  depending  in  part  upon 
the  retention  of  the  heat,  and  in  part  upon  the  retention  of  the 
aqueous  vapor,  they  would  consequently  remain  in  a  liquid  state 
much  longer  than  the  lavas  of  sub-aerial  volcanoes.  They  would 
therefore  take  a  more  highly  crystalline  form.  All  the  loose 
materials  thrown  out  during  the  eruption  would  be  removed  by 
oceanic  currents,  and  hence  no  cone  would  be  built  up  around  the 
orifice  of  eruption.  We  may  therefore  regard  the  trappean  rocks 
as  the  lavas  of  submarine  volcanoes.  The  present  volcanoes  of 
this  kind  are  necessarily  producing  the  same  kind  of  rocks,  though 
there  will  be  no  other  proof  that  they  exist,  except  the  existence 
of  the  volcano,  till  the  bed  of  the  sea  becomes  dry  land. 

The  granitic  rocks  are  also  the  product  of  igneous  causes. 
Granite  is  the  most  abundant  of  these  crystalline  rocks ;  and  the 
others,  such  as  crystalline  limestone,  are  so  intimately  associated 


IGNEOUS   CAUSES.  139 

with  granite  that  they  must  have  had  the  same  origin.  Gran- 
ite is  everywhere  found  to  send  off  dikes  into  the  overlying  rocks, 
and  must  therefore  have  been  in  a  state  of  fusion ;  that  is,  it  must 
have  existed  as  lava  beneath  the  surface.  It  is  obvious  that  fluid 
lava  always  exists  in  great  quantity  beneath  areas  of  energetic 
volcanic  activity. 

Portions  of  this  lava  must  in  succession  take  the  solid  form. 
Wherever  the  surface  is  elevated  along  a  line  of  fracture,  the  lava 
which  is  accumulated  beneath  rises  above  the  level  of  the  general 
reservoir  of  lava,  and  will  therefore  part  with  its  heat  more  rapidly. 
Pn  cooling,  it  becomes  the  granitic  nucleus  of  the  mountain.  We 
ought  also  to  suppose  that,  by  the  extremely  slow  process  of  the 
transmission  of  heat  to  the  surface,  the  crust  of  the  earth  is  every- 
where increasing  in  thickness ;  that  is,  the  upper  portion  of  the 
great  lava  mass  is  solidifying. 

Sir  James  Hall  has  shown,  by  experiment,  that  earthy  sub- 
stances, reduced  to  a  state  of  fusion,  become  more  highly  crystal- 
line as  they  are  allowed  to  cool  more  slowly,  and  are  subjected  to 
greater  pressure.  It  is  difficult  to  conceive  of  these  conditions 
existing  in  a  higher  degree  than  they  do  in  the  cooling  masses  of 
lava  below  the  stratified  rocks.  These  lavas  must  therefore  take  the 
highly  crystalline  form  which  the  granitic  rocks  are  found  to  have. 

All  the  igneous  rocks  have  therefore  existed  as  subterranean 
lavas.  The  volcanic  rocks  have  become  vitreous,  the  granitic  are 
crystalline,  and  the  trappean  are  intermediate  in  structure,  co- 
inciding with  the  circumstances  of  pressure  and  rate  of  cooling 
under  which  they  have  severally  been  formed. 

5.  The  Elevation  of  Mountains  is  another  result  of  volcanic 
action.  The  height  of  mountains  depends,  in  part,  upon  general 
elevation.  Yet  there  is  a  different  action,  upon  which  the  exist- 
ence of  the  mountain,  as  such,  depends.  Whenever  igneous  action 
becomes  intense  under  any  portion  of  the  earth's  surface,  and  the 
elastic  force  greater  than'  the  repressive,  the  solid  crust  will  be 
broken  and  raised  up,  and  along  this  line  of  fracture  the  lava  will 
rise  above  its  general  level  elsewhere.  This  lava,  thus  lifted  out  of 


140  IGNEOUS   CAUSES. 

the  general  mass,  in  time  solidifies,  and  forms  the  nucleus  of  a 
mountain.  At  successive  periods  the  elevating  force  is  renewed, 
and  adds  somewhat  to  the  mountain  mass  before  supplied.  In  this 
way  the  mountain  is  ultimately  formed. 

So  far  as  observations  have  been  made,  the  elevation '  of  moun- 
tains seems  not  to  be  gradual,  but  spasmodic ;  and  yet  the  elevating 
force  probably  accumulates  constantly  and  uniformly.  The  repress- 
ing force  consists  of  the  weight  of  the  strata  above,  which  may  be 
regarded  as  constant,  and  their  strength,  which  is  variable.  When 
the  elevating  force  becomes  greater  than  both  the  repressing  forces, 
the  crust  is  fractured.  The  strength  of  the  strata  then  becomes 
nothing,  and  the  repressing  force  is  the  weight  alone.  The  elastic 
mass  below  at  once  expands,  and  the  requisite  space  is  furnished  by 
the  uplifting  of  the  strata  along  the  line  of  fracture.  As  the  ridge 
of  lava  which  fills  this  additional  space  cools,  it  recloses,  in  part, 
the  original  fracture,  and  the  repressing  force  again  consists  of  the 
two  elements,  —  weight  and  strength.  There  will  therefore  be  no 
further  elevation  till  the  elevating  force  is  again  superior  to  these 
two  forces.  Thus  the  elevating  force,  though  it  may  accumulate 
at  a  uniform  rate,  will  manifest  itself  only  at  considerable  intervals. 

As  the  accumulation  of  lava  along  the  line  of  fracture  is  the 
cause  of  the  upheaval,  every  mountain  must  have  a  central  gran- 
itic axis.  Sometimes  this  granitic  mass  is  pushed  up  through  the 
fissure,  as  in  the  case  of  Mont  Blanc.  At  other  times,  the  stratified 
rock,  which  formed  the  original  surface,  is  carried  up  so  as  to  form 
the  surface  rock  nearly  to  the  top.  In  either  case,  the  strata  are 
lifted  along  the  line  of  fracture,  and  left  in  an  Inclined  position. 
In  this  position  the  older  rocks  are  always  found,  wherever  there 
has  been  any  considerable  amount  of  igneous  disturbance. 

In  some  instances,  the  additional  space  required  by  the  expan- 
sion of  the  igneous  mass  below  is  furnished,  not  by  the  uplifting 
of  the  strata,  but  by  .their  compression  into  folds  between  two  lines 
of  upheaval.  The  igneous  rock  is  elevated  but  little  above  the 
stratified  through  which  it  had  burst ;  but  the  stratified  rocks  have 
taken  the  undulatory  form,  and  the  widening  of  the  igneous  mass 


IGNEOUS   CAUSES. 


141 


along  the  lines  of  fracture  lias  compressed  the  undulations,  until 
the  planes  of  the  strata  have  become  vertical.  Fig.  82  will  give 
an  idea  of  the  successive  changes  by  which  the  vertical  position  of 
the  strata  has  been  produced. 

Fig.  82. 


The  force  by  which  mountains  are  elevated  being  the  elasticity  of 
the  vapor  diffused  through  the  subjacent  lava,  it  may  happen,  if  the 
lava  have  a  high  degree  of  fluidity,  that  this  vapor  will  collect  in  large 
masses,  and  rise  as  far  as  the  lava  is  in  a  fluid  state.  The  irregular 


142  IGNEOUS   CAUSES. 

flow  of  lava  from  craters  during  an  eruption  is  undoubtedly  due 
to  the  rapid  ascent  of  such  steam  bubbles  through  the  lava.  Such 
an  accumulation  of  vapor  under  a  mountain  mass,  if  it  cannot 
escape,  would  support  it  as  long  as  the  temperature  remained 
unchanged.  But,  upon  a  reduction  of  temperature,  the  mass  which 
had  been  upheaved  by  it  would  be  unsupported,  and  liable  at  any 
time  to  sink.  Instances  of  subsidence  on  a  comparatively  small 
scale  will  admit  of  explanation  in  this  way.  Papandayang,  one 
of  the  loftiest  volcanic  mountains  of  Java,  sunk  down  four  thousand 
feet  in  the  year  1772.  The  area  engulfed  was  sixteen  miles  long 
and  six  broad.  The  crater  of  Kilauea,  in  one  of  the  Sandwich 
Islands,  was  evidently  formed  in  this  way.  It  is  situated  on  the 
side  of  a  mountain,  and  consists  of  a  chasm  eight  miles  in  cir- 
cumference and  a  thousand  feet  in  depth.  Liquid  lava  can  always 
be  seen  boiling  in  the  small  craters  at  the  bottom ;  and  at  times  it 
rises  so  as  to  overflow  them,  and  fill  the  chasm  to  within  four  hundred 
feet  of  the  top,  when  lateral  subterranean  passages  are  opened,  by 
which  it  is  discharged.  The  same  explanation  —  a  depression  of 
the  central  portion — may  be  given  of  the  formation  of  the  large 
craters  in  the  Canary  and  Grecian  islands.  It  is  also  probable  that 
Lake  Avernus  and  others,  in  Italy,  and  some  in  Germany,  have 
had  a  similar  origin. 

The  subsidence  of  Papandayang  is  of  importance  as  a  historical 
fact ;  and  it  is  not  at  all  unreasonable  to  suppose  that  larger  chasms 
of  great  depth  were  also  sudden  subsidences  of  a  similar  character. 
Lake  Superior  has  a  depth  considerably  greater  than  the  elevation 
of  its  surface  above  the  level  of  the  sea.  The  bottom  of  the  Dead 
Sea  is  two  thousand  six  hundred  feet  below  the  surface  of  the 
Mediterranean.  And  at  one  place  in  the  Atlantic  Ocean  a  sound- 
ing was  attempted  with  more  than  six  miles  of  line,  without  reach- 
ing bottom.  These  sunken  areas,  however,  though  of  great  extent, 
occupy  only  an  insignificant  portion  of  the  entire  surface  of  the 
earth. 

6.  The  Elevation  of  Continents.  —  The  causes  of  change  of 
level  which  have  been  given  will  not  explain  those  slow  vertical 


IGNEOUS  CAUSES.  143 

movements  which  are  now  taking  place  in  Greenland  and  the  north 
of  Europe,  or  those  by  which  the  present  continents  have  been 
elevated  and  the  bed  of  the  sea  depressed.  Any  cause  which  will 
account  for  these  movements  must  be  one  operating  for  long 
periods,  under  large  areas,  and  with  great  uniformity. 

The  cause  which  fulfils  all  these  conditions  most  satisfactorily  is 
a  variation  of  temperature  in  the  mass  of  rock  underlying  the 
portion  of  the  surface  whose  level  is  changing.  It  has  before  been 
shown  that  the  temperature  increases  as  we  descend  below  the  sur- 
face ;  but  there  is  also  reason  to  suppose  that  it  undergoes  great 
variations.  The  volcanic  grits  interstratified  with  the  silurian 
rocks  of  England  show  that  at  the  silurian  period  volcanic  fires 
were  active  below  that  portion  of  the  surface.  When  the  early 
fossiliferous  rocks  of  this  country  were  deposited,  the  Alleghany 
Mountains  had  not  been  elevated ;  but  before  the  tertiary  period 
they  had  taken  nearly  their  present  form.  Some  portion  of  the 
intermediate  period  was  therefore  one  of  volcanic  upheaval.  The 
trappean  rocks  are  also  evidence  of  intense  volcanic  action  existing 
here.  France,  during  the  tertiary  period,  was  a  highly  volcanic 
country ;  but  all  volcanic  activity  has  now  subsided.  The  Andes 
have  been  mostly  elevated  since  the  tertiary  period,  and  are  still 
rising.  It  is  evident,  then,  that  at  different  periods  volcanic  heat 
may  vary  from  its  highest  to  its  least  degree  of  activity,  below 
any  portion  of  the  earth's  surface. 

This  variation  of  temperature  must  be  followed  by  variation  of 
volume  of  the  earth's  crust ;  that  is,  it  must  produce  expansion  or 
contraction.  Experiments  have  been  made,  under  the  direction  of 
the  United  States  government,  to  determine  the  expansion  of  the 
several  kinds  of  rock  used  in  our  public  works.  It  was  found  that 
granite  expands  nearly  one  two  hundred  thousandth  of  its  length 
for  every  degree  of  increased  temperature,  limestone  somewhat 
more  than  that,  and  sandstone  about  twice  as  much.  Taking  the 
expansion  of  the  granite  as  the  basis  of  calculation,  and  supposing 
the  crust  for  a  hundred  miles  in  thickness  to  be  undergoing  change 
of  temperature,  there  would  be  a  resulting  difference  of  level 


144  IGNEOUS   CAUSES. 

exceeding  two  and  a  half  feet  for  each  degree  of  change  in  temper- 
ature, or  more  than  two  thousand  five  hundred  feet  for  a  change  of 
one  thousand  degrees. 

This  calculation  is  made  upon  the  supposition  that  the  law  of 
expansion  is  the  same  for  all  temperatures,  and  that  no  new  con- 
ditions are  introduced  at  high  temperatures  by  the  presence  of 
aqueous  particles.  We  know,  however,  that  solids  expand  more 
rapidly  at  high  temperatures  than  at  low,  and  the  elasticity  of 
aqueous  vapor  at  high  temperatures  must  increase  the  rate  of 
expansion  of  the  rock  through  which  it  is  diffused.  Although  we 
are  not  able  to  introduce,  numerically,  the  effect  of  these  two  cir- 
cumstances, yet  it  is  obvious  that  they  must  be  considerable. 

The  mean  elevation  of  land  above  the  level  of  the  sea  is  about 
nine  hundred  feet,  the  mountain  masses  above  that  level  not  being 
included ;  and  the  estimated  mean  depth  of  the  ocean,  not  includ- 
ing its  chasms,  does  not  exceed  two  thousand  six  hundred  feet. 
The'  total  elevation  of  the  continental  masses,  for  which  it  is  neces- 
sary to  account,  does  not  therefore  exceed  three  thousand  five  hun- 
dred feet.  This  amount  of  vertical  movement  may  evidently  be 
produced  by  the  expansion  and  contraction  resulting  from  changes 
of  temperature. 

These  changes  of  level  must,  however,  be  very  gradual.  Any 
diminution  of  temperature  must  result  from  the  transfer  of  heat  to 
the  surface  ;  and  the  conducting  power  of  rocks  is  very  imperfect. 
The  lava  in  a  crater  is  often  so  cooled  on  the  surface  that  it  can  be 
walked  on,  while  but  a  few  feet  below  it  is  still  liquid.  Lava  cur- 
rents continue  in  gradual  motion  long  after  the  surface  is  nearly 
cold.  This  was  the  case  with  one  of  the  currents  from  ^Etna  for 
more  than  nine  months  after  its  eruption,  and  with  another  for 
ten  years.  Humboldt  visited  Jorullo  forty  years  after  it  was 
thrown  up,  when  the  lava  around  the  mountain  was  still  in  a 
heated  state,  the  temperature  in  the  fissures  being  on  the  decrease 
from  year  to  year ;  but  twenty  years  after  its  ejection  the  heat 
was  still  sufficient  to  light  a  cigar  at  the  depth  of  a  few  inches. 
If  so  long  a  period  is  insufficient  to  solidify  a  comparatively  small 


IGNEOUS  CAUSES.  145 

quantity  of  melted  rock  when  the  circumstances  for  cooling  are 
most  favorable,  we  may  well  suppose  that  centuries  would  be 
required  to  abstract  sufficient  heat  from  the  earth's  crust  to  pro- 
duce any  material  change  in  the  areas  of  continents. 

If  this  account  of  the  elevation  and  subsidence  of  continents  is 
correct,  it  would  seem  that  they  ought  to  be  constantly  undergoing 
change  of  level.  And  their  apparent  stability  may  be  regarded 
as  an  objection  to  it.  If  in  any  place  there  is  absolutely  no  verti- 
cal movement,  then  those  conditions  must  exist  in  which,  for  the 
time  being,  there  is  no  change  of  temperature. 

But  it  is  doubtful  whether  there  ever  is  absolute  stability  of 
any  portion  of  the  surface  for  long  periods  of  time.  Of  the  minor 
vertical  movements  of  the  interior  of  continents,  there  can,  from 
the  nature  of  the  case,  be  no  evidence  whatever.  Changes  of 
level,  where  they  are  known  to  be  taking  place,  are  so  slow,  that 
they  are  hardly  perceptible  in  the  period  of  a  human  life.  Such 
changes  had  been  going  on  for  centuries  in  Sweden  before  they  were 
suspected.  As  accurate  observations  have  increased  in  number, 
and  historical  records  become  available,  it  is  becoming  known  that 
a  very  large  amount  of  the  seaboard  is  undergoing  change  of  level. 
It  becomes  probable,  then,  that  these  extremely  slow  changes  of 
level  are  constantly  and  everywhere  taking  place. 

That  portion  of  the  crust  of  the  earth  constituting  the  present 
continents,  being  further  removed  from  the  centre,  would  part  with 
its  heat  more  rapidly,  and  receive  heat  from  the  central  mass  more 
slowly,  than  that  portion  which  at  present  constitutes  the  bed  of 
the  sea.  The  continents  are  therefore  in  a  situation  to  undergo 
contraction  and  depression,  and  the  bed  of  the  sea  is  most  favorably 
situated  for  rising.  If  the  distribution  of  water  through  the  mass 
has  any  influence  in  promoting  its  expansion,  then  the  bed  of  the 
sea  would  receive  this  supply  most  abundantly,  and  the  continents 
the  least  so.  We  see,  then,  in  nature,  those  provisions  for  an 
alteration  of  level,  which,  from  the  character  of  the  several  rock 
formations,  we  know  to  have  taken  place.  When  any  portion  of 
the  earth's  surface  is  covered  with  the  sea,  the  conditions  exist 
13 


OF  THE 

UNIVERSITY 


146  IGNEOUS  CAUSES. 

•which  will  at  length  elevate  it.  When  it  becomes  dry  land,  the 
conditions  exist  which  will  in  time  depress  it  below  the  level  of  the 
ocean.  Hence,  those  impressions  in  regard  to  the  land,  as  stable 
beyond  the  possibility  of  change,  we  ought  to  abandon ;  and  those 
vertical  movements,  which,  when  proved,  we  are  accustomed  to 
regard  as  extraordinary,  we  shall,  at  length,  consider  as  only  par- 
ticular instances  of  one  of  the  most  general  laws  of  nature. 

7.  Variations  of  Climate.  —  The  only  sources  of  heat  by  which 
climate  can  be  affected  are  the  sun  and  the  heated  interior  of  the 
earth. 

If  the  former  melted  condition  of  the  entire  mass  of  the  earth 
be  assumed,  the  temperature  of  the  surface  must  have  been  in- 
creased, by  conduction  of  heat  from  within,  for  long  periods  after 
the  superficial  stratum  had  become  solid.  It  is,  however,  suscepti- 
ble of  proof,  that  the  present  climates  are  not  sensibly  affected  by 
interior  heat,  though  at  a  little  more  than  a  mile  below  the  surface 
the  temperature  is  equal  to  that  of  boiling  water.  At  any  time, 
therefore,  after  the  waters  had  become  condensed,  collected  into 
oceans,  and  become  sufficiently  cool  to  support  the  animal  life  of 
which  the  remains  are  now  found,  it  is  not  probable  that  the  cli- 
mate was,  to  any  considerable  extent,  influenced  by  the  heat  con- 
ducted from  the  interior. 

Still,  there  have  been  great  changes  of  climate  since  those  early 
organic  forms  existed ;  and,  since  we  have  no  ground  for  supposing 
that  the  temperature  of  the  sun's  rays  has  suffered  any  reduction, 
we  have  to  inquire  whether  the  means  of  retaining  the  heat  from 
the  sun  could  at  any  time  have  been  different.  The  relative  posi- 
tion of  land  and  water  depends,  as  we  have  seen,  upon  igneous 
causes,  and  has  been  very  different  at  different  times.  We  shall 
find  that  climate  must  have  been  greatly  modified  by  these  changes ; 
for  the  land  radiates  and  absorbs  heat  freely,  and  water  possesses 
this  power  in  a  very  low  degree. 

Let  us  suppose  the  zone  comprised  between  the  tropics  to  be 
occupied  by  land,  and  the  portions  without  these  limits  to  be 
covered  with  water.  Under  these  conditions,  the  land,  having  a 


IGNEOUS  CAUSES.  147 

nearly  vertical  sun  the  whole  time,  would  accumulate  heat  to  a 
degree  scarcely  compatible  with  the  existence  of  animal  life.  This 
is  sufficiently  proved  by  the  oppressive  tropical  climates  of  the 
present  time,  influenced  as  they  are  by  polar  lands  and  contiguous 
seas. 

Under  the  same  conditions,  the  sea  would  be  heated  by  contact 
with  the  land,  and  the  heat  would  be  distributed  by  marine  cur- 
rents to  the  polar  regions.  But  the  water  thus  distributed  would 
not  part  with  its  heat,  because  it  has  but  little  radiating  power, 
and  nowhere  comes  in  contact  with  polar  land.  It  follows,  then, 
that  both  land  and  water  would  be  subjected  to  a  very  high 
temperature. 

But,  if  we  suppose  the  land  confined  to  the  polar  regions,  and 
the  sea  to  the  equatorial,  the  opposite  results  would  follow.  The 
equatorial  sea  would  absorb  but  a  small  proportion  of  the  solar  heat 
which  would  be  thrown  upon  it.  The  land  would  receive  the  sun's 
rays  too  obliquely  to  receive  much  elevation  of  temperature,  as  the 
present  polar  climates  show.  Hence,  the  temperature  of  the  earth 
would  differ  but  little  from  that  of  the  planetary  spaces,  which  is 
fifty-eight  degrees  below  zero,  a  temperature  too  low  to  allow  of 
any  considerable  development  of  organic  life. 

These  are  the  conclusions  to  which  we  are  led  by  considering 
the  different  powers  of  land  and  water  to  absorb  and  radiate  heat, 
and  we  shall  find  that  the  existing  climates  are  in  accordance  with 
these  conclusions.  America  has  a  lower  temperature  than  Europe 
in  the  same  latitudes.  It  has  also  a  smaller  proportion  of  land  in 
the  equatorial  regions,  and  a  greater  proportion  in  the  north  polar 
regions.  The  eastern  continent  is  colder  in  Asia  than  in  Europe 
in  the  same  latitudes.  It  has  also  less  equatorial  and  more  polar 
land.  The  southern  is  colder  than  the  northern  hemisphere  at 
equal  distances  from  the  equator.  There  is  also  less  land  near  the 
equator  on  the  south  side,  and  probably  as  much  land  around  the 
south  as  the  north  pole. 

Hence,  we  see  that  there  may  have  been  such  a  relation  of  land 
and  water  as  to  account  for  all  the  variations  of  iemperature  which 


148  IGNEOUS  CAUSES. 

are  known  to  have  existed.  We  cannot  say  that  such  actually  has 
been  the  case.  We  can  tell,  with  some  degree  of  accuracy,  what 
portions  of  the  present  continents  were  land  at  the  several  geologi- 
cal periods ;  but  three-fourths  of  the  surface  of  the  earth  is  covered 
with  water,  and  of  the  condition  of  this  portion  during  those 
periods  we  have  no  means  even  of  conjecturing.  We  can  only 
say,  that,  by  the  operation  of  known  causes,  the  relative  position  of 
land  and  water  may  have  been  such  as  to  produce  the  climates 
known  to  have  existed  at  former  periods  of  the  history  of  the 
earth. 


INDEX. 


A. 


Page 


Abundance  of  vegetable  products 

of  the  coal  period, 59 

Accumulation  of  vegetable  mat- 
ter/  117 

Actinolite, 15 

Action  of  internal  heat,  .   .   .   .128 

Action  of  waves, 107 

in  forming  harbors,    ...  108 
Advantages  of  geological  chang- 
es,      91 

^Etna, 26,  73 

Agate, 14 

Age  of  rocks,  doubtful  — 

from  change  of  lithological 

character, 61 

from  distance, 61 

from  disturbance,    ....    61 
Alternation  of  coarse  and  fine 

material, 115 

Aluminium, 12 

Amethyst, 14 

Amygdaloidal  structure,  ...  17 
Ancienj;  volcanic  rocks, ....  29 
Andes,  granite  veins  in,  ...  25 

Angle  of  inclination, 71 

Anoplotherium, 55 

Anticlinal  axis, .    71 

A.queous  causes,      103 

13* 


Aqueo-glacial  action,  .  .  .  .120 
Argillaceous  schist,  ....  20, 31 
Arrangement  of  materials  in  the 

crust  of  the  earth, 21 

Artesian  wells, 92 

Asbestus,      15 

Atmospheric  causes, 95 

Atolls, 81 

Augite, 15 

Auvergne,  volcanic  district  of,     28 

B. 

Basalt, 18 

Bed  of  the  sea  — 

sunken  areas  in  the,  .   .   .  142 
why  elevated, 145 

Belemnites, 52 

Breccia,    .   .    . 19 

Brine  springs  — 

in  Silurian  rocks,    ....    35 
in  the  carboniferous  forma- 
tion,   43 

in  the  new  red  sandstone,     47 

C. 

Calamite, 47 

Calcium, 13 


150 


INDEX. 


Page 

Cambrian  system, 32 

Carbon, 11 

Carbonate  of  lime, 15 

Carbonate  of  magnesia,      ...  19 
Carbonic  acid  a  cause  of  dis- 
integration of  rocks,    ....  95 
Carboniferous  formation,  ...  39 
essential  to  national  wealth,  43 

extent  of, 47 

a  prospective  arrangement,  43 

faults  in, 41 

not  always   disturbed    by 

faults, 42 

Carboniferous  limestone,    ...  39 
sometimes  becomes  a  coal- 
bearing  rock,  .....  42 

fossils  of  the, 40 

Carnelian, 14 

Cause  of  internal  heat,  .    .    .   .128 
Cause  of  stratification,   .    .    .    .114 

Caverns, 69 

Cephalaspis, 39 

Cephalopoda, 36 

in  oolite, 50 

Chalcedony, 14 

Chalk, 52 

Changes  of  climate, 88 

how  produced, 146 

Changes   in  the   crust  of   the 

earth, 67 

of    temperature    a    disin- 
tegrating agent,  ....  96 

at  the  surface, 85 

Chemical  action, 97 

in  solids, 99 

in  crystallization,    ....  97 

Chlorine, 12 

Chlorite, 15 

Chlorite  slate, 32 

Classification  of  rocks,   ....  21 


Clay, 19 

Clay  slate, 20 

Cleavage  structure,    .   .   .   .  68,  98 

Coal, 16 

varieties  of, 42 

mode  of  quarrying,     ...    42 

origin  of, 116 

conversion     of     vegetable 

matter  into, 117 

now  forming, 118 

Coal  measures, 131 

fossils  of  the, 44 

Coal  plants,  tropical  character 

of,      88 

Coal  and  iron  associated,  ...    43 

Clouded  marble, 33 

Columnar  structure,  ....  18,  99 

Compact  limestone, 19 

Concretionary  formations,     .    .    99 

Conglomerate, 19 

of  old  red  sandstone,  ...    38 
Connecticut  valley  — 

one  of  denudation,      ...    87 
trap  of,     ........    30 

Continents  — 

mean  elevation  of,  ....  144 

total  elevation  of,    ....  144 

elevated  gradually,    .    .    .  144 

why  depressed, 145 

Contorted  strata, 72 

Coral  formation,      ....   81,102 

extent  of, 102 

Coral  reefs  — 

fringing, 81 

barrier, 81 

Coral  rag, 49 

Corals  in  silurian  rocks,  ...  35 
Copper  mines  of  Lake  Superior,  47 
Creation,  a  progressive  work,  .  62 
Cretaceous  formation,  ....  52 


INDEX. 


151 


Cretaceous  formation,  fossils  of,    52 

geographical  range  of,   .    .    53 

Crinoidea  in  Silurian  rocks,  .   .    36 

Crust  of  the  earth, 16 

expansion  and  contraction 
of,     ...  f*  .  *.  .   .  143 

D. 

Delta  deposits, 114 

Denudation  of  igneous  rocks,    .    85 
Denudation     of     sedimentary 

rocks, 85 

Denudation  produced  by  earth- 
quake waves, 136 

Deposition  of  sediment,  .   .   .   .113 

Diluvium, 20 

Dike, 69,133 

Divisional  planes, 68,  98 

Dolomite, 13, 19 

Drift, 20 

extent  of, 53 

connected  with  striated  sur- 
face of  the  rocks,     ...    54 
connected    with      subsid- 
ence,   54,126 

E. 

Earth  in  a  state  of  change,    .   .    97 

Earthquakes, 130 

wave-like  motion  of,   .   .    .136 
Earthquake  waves,  rocks  shiv- 
ered by,   137 

Effect  of  atmospheric  agencies,  95 
Electrical  discharges,  effect  of,  96 
Elementary  substances,  .  ,  .  11 
Elevation  and  subsidence,  .  .  73 
Elevation  and  subsidence  sev- 
eral times  repeated,  ....  82 


Elevation  of  mountains,     ...    73 

cause  of, 139 

spasmodic, 140 

gradual, 75 

Elevation  of  different  mountains 

at  different  times, 75 

Elevation  of  continents,     ...    76 

cause  of, 142 

Elevation  of  North  America,     .    76 
Elevation  of  the  coast  of  Maine,    76 

Elevation  of  Europe, 77 

Elevation  of  South  America,     .    78 
Encrinites, 36, 50 


P. 

Fault, .41,69 

Felspar, 14 

Filling  up  of  lakes, 76 

Fingal's  cave, 30 

Fissile  structure,  origin  of,   .    .    68 

Flint, 14 

in  chalk, 52, 99 

Fluorine, 12 

Folded  axes, 61,  72 

Formation  of  soils, 87 

Fossils  — 

definition  of, 57 

how  preserved, 57 

mineralization  of,    ....    58 

use  of, 60 

order  in  which  animals  ap- 
peared, shown  by,  ...    58 
animal  and  vegetable,  cre- 
ated together,     ....    59 
as  a  record  of  climate,  88, 101 

Fossiliferous  rocks, 32 

classification  of, 32 

Fractures, 42,  68,  130 

opening  downward,    .    .    .133 


152 


INDEX. 


G. 

Page 

Garnet, 16 

Geological  causes,  how  far  uni- 
form,  94 

Geological  causes,  slow  opera- 
tion of, 95 

Geological  investigations  aided 

by  displacement  of  strata,     .    92 

Geological  periods,  prolonged,   .    63 

shown  by  amount  of  strata,    63 

shown     by    duration     of 

species,     .......    64 

shown  by   amount  of  or- 
ganic matter, 64 

shown  by  microscopic  ac- 
cumulations,   .    .    .    •   .    65 
Geology  and  Revelation,    ...    65 

Giant's  Causeway, 30 

Glacial  period, 90 

Glacial  theory, 124 

Glaciers  — 

how  formed, 120 

cause  of  motion, 121 

when  they  decrease,  .  .  .122 
earthy  matter  on  them,  .  122 
lateral  moraines,  .  .  .  .123 
surfaces  grooved  by,  .  .  .  123 
terminal  moraines,  .  .  .123 

Gneiss, 18,  30 

Gorge, 69 

Graham  Island, 131 

Granite, 16 

varieties  of, 16 

thickness  of, 23 

structure  of, 23 

formation  of, 67 

igneous  origin  of,    .    .    .    .138 

Granite  veins, 24 

in  granite, 24 

Granite  of  different  ages,  ...    25 


Paga 

Grranitic  axes  of  mountains,  24, 140 

Greensand, 19,  52 

Greenstone, 18,  30 

Grooved  surfaces  of  rock,  54,87,126 

Gypsum, 15 

in  new  red  sandstone,    .    .    47 
beds,  how  produced,  ...    99 


H. 

Hall,      Sir      James,      experi- 
ments,   135, 139 

Heterocercal  tails  of  fishes,    .    .    48 

Homocercal  tails, 48 

Hornblende, 14 

Hornblende  slate, 20,32 

Hydrogen, 11 

Hypers  thene, 15 

Hypersthene  rock, 17, 25 


I. 

Icebergs  — 

how  formed, 124 

earthy  materials  in,   ...  125 

motion  of, 125 

size  and  number  of,  ...  125 
effect  in  distributing  drift,  126 
grooving  the  surface,  .  .126 

Iceland  spar, 19 

Iceland,  volcanic  eruption  in,  .    26 

Ichthyosaurus, 50 

Igneous  causes, 127 

Iguanodon, 51 

Inclined  position  of  strata  pro- 
duced by  upheaval, .    .    .  70, 140 
Increase  of  temperature  below 

the  surface,  . 127 

Iron, 12 


INDEX 


153 


Jasper, 14 

Jorullo, 131 

K. 
Kilauea,       26, 142 


Lakes,  filling  up  of,    ...  76,  113 

Lava, 17,25,137 

varieties  of, 25 

tertiary, 27 

elastic    vapors    contained 

in, 130,132 

great  quantity  of  modern,    26 
Lead-bearing  strata,  .   .   .   .  35,  40 

Lepidodendron, 46 

Lias, 19,49 

Limestone, 15, 19 

as  a  primary  rock,  ....    25 

metalliferous, 40 

Local  changes  of  climate,  ...    90 


M. 

Magnesian  limestone,    .   .   .19,  47 

Magnesium, 13 

Mammoth, •   •    56 

Man  — 

recently  created,     ....    59 
as  an  agent  in  producing 

geological  changes,  .  .  101 
impressions  of  the  feet  of,  .  59 
skeleton  of  from  Guada- 

loupe, 59 

Manganese,      12 

Marine  currents, 108 


Marine  currents,  cause  of,     .   .  109 
abrading  power  of,  .   .   .   .111 

Marl, 19 

Mastodon, 55 

VIegatherium,      55 

tfetamorphic  changes,    ....    67 

Hetamorphic  rocks, 30 

amount  of, 67 

origin  of,  .......   .134 

order  of  superposition,   .   .    31 
upper  limit  variable,  ...    13 

localities  of, 32 

Metallic  ores, 92 

Mica, 14 

slate, 18,31 

Millstone  grit, 40 

fossils  of, 41 

becomes  coal  measures,  .  .   42 
Mineral  — 

definition  of,    ......    13 

Mineral  veins, 69 

formation  of, 100 

Modern  formation, 57 

•why  but  little  known,    .   .    67 

fossils, 57 

Moisture  of  the  atmosphere  a 
disintegrating  agent,  ....    96 

Monte  Nuovo, 26 

Mount  Loa,  eruption  of,    ...    26 


N. 

Neocomian  system, 52 

New  red  sandstone, 47 

fossils  of, 48 

ores  of, 47 

geographical  range  of,  .   .  49 

Niagara  Falls,  how  preserved,  .  106 

Nitrogen,      11 

Nummulite  rock, 54 


154 


INDEX. 


o. 

Pa-e 

Oceanic  mountains, 75 

Ocean  level,  nearly  permanent,  74 

Old  red  sandstone, 38 

fossils  of, 39 

extent  of, 39 

Oolite, 19 

Oolitic  structure, 49 

Oolite  system, 49 

calcareous, 49 

fossils  of, 50 

localities  of, 51 

Opal, 14 

Organic  causes, 101 

Orthoceras, 36, 40 

Outcrop, 71 

Oxide  of  iron, 16 

Oxygen, 11 


P. 

Paleotherium, 55 

Papandayang, 95,  142 

Permian  system, 47 

Plesiosaurus, 51 

Porphyritic  structure,    ....  17 

Potassium, 12 

Primary  limestone, 19 

Pterodactyle, 51 

Pumice-stone, 17 

Pyroxene, 15 

Q. 

Quartz, 14 

rock, 32 


R. 

Raindrops,  impressions  of,    .   .    48 


Pa^e 

Raised  beaches, 76 

Ravine, 69 

Recent  elevation  in  Europe,  .   .    79 

Recent  formation, 57 

Ripple  marks, 48 

Rivers  — 

beds  of,  raised, 113 

continued  into  the  sea,  .    .112 
abrading  action  of,     ...  105 
abrading  action  of  promoted 
by  foreign  substances,    .  106 

Rock  crystal, 14 

Rock  salt, 16,  118 

Rocks,  denned, 16 

Rose  quartz, 14 


S. 

Saccharine  limestone,    .   .   .19, 32 

Saliferous  system, 47 

Saline  properties  of  the  ocean, 

how  obtained, 104 

Salt  beds  — 

where  found, 118 

how  formed, 119 

Sandstone, 19 

Schorl,      16 

Scoria3, 17,  133 

Sediment  — 

amount  of  in  rivers,  .  .   .  107 

deposition  of, 113 

sorted  by  rivers,     .   .   .   .112 

Selenite, 16 

Serpentine, 15 

a  primary  rock, 25 

Shale, 20 

Siberia,  remains  of  elephants  in,    89 

Sigillaria, 45 

Silicium, 12 

Sinking  of  wharves,  towns,  &c.,    79 


UNIVERSITY    -I 
/ 

INDEX. 


155 


Silurian  system, 34 

tabular  arrangement  of,    .    34 

divisions  of, 35 

fossils  of, .    36 

geographical  range,    .   .   .    38 
Slaty  structure  — 

in  the    gold  washings  of 

Chili, 98 

produced  by  electric  cur- 
rents,     98 

Slope  of  mountains, 73 

Soapstone, 15 

Sodium, 13 

Solidification  of  rocks,    .   .  68,119 
Soluble  materials  of  rocks,   .    .  103 
Solution  of  mineral  substances — 
promoted  by  heat,  ....  104 
promoted  by  an  alkali,  .   .  104 
promoted  by  carbonic  acid,  105 
Sources  of  the  sedimentary  ma- 
terials,       103 

Sources    of    the    sediment   of 

rivers, 97 

Species  — 

disappearance  of,    ....    62 
causes  of  the  disappearance 

of, 62 

Springs, 92 

Stability  of  continents  only  ap- 
parent,      145 

Statuary  marble, 19 

Stigmaria, 44 

Strata  — 

horizontal, 70,  115 

permeable  and  impermea- 
ble,    92 

irregular,  how  produced,  .115 

Striated  surfaces, 87 

Submerged  forests, 79 

Subsidence  of  land,    ...  79, 145 


Paje 

Subsidence  of  land  in  Green- 
land,   82 

Subsidence  and  elevation  in  the 

Pacific, 80 

Sulphate  of  lime, 15 

Sulphur, 12 

Sun-cracks, 48 

Sunken  areas, 142 

Syenite, 17 

Synclinal  axes, 72 


T. 

Taconic  system, 32 

Talc,     15 

Talcose  slate, 20,32 

Temperature  at  great  depths,  .  127 
Temple  of  Jupiter  Serapis,    .   .    83 

Tertiary  system, 53 

age,  how  determined,    .   .    53 

fossils, 64 

divisions  of, 53 

geographical  range,   .  ,   .    56 
Tilestones  of  the  old  red  sand- 
stone,     38 

Trachyte, 18 

Tracks  in  new  red  sandstone,  .    49 
Transportation  of  sediment,  .    .111 

Trappean  rocks, 17 

localities  of, 29 

Tremolite, 15 

Trias, 47 

Trilobite,      37,40 


V. 

Valley, 69 

of  elevation, 71,76 

of  subsidence, 72 


156 


INDEX. 


Pa-e 

Valley  of  denudation,     ....    87 

Vein  of  segregation, 69 

Verd-antique  marble,     ....    15 

Volcanic  rocks, 17,25 

of  different  ages,     ....    26 
in  what  states  ejected,   .   .    25 
Volcanic     mountains,    dimen- 
sions,     26 

Volcanic  activity  — 

regions  of, 129 

water  essential  to,  .   .   .   .130 
Volcanic  cones  — 

formation  of, 131 

lateral, 132 

Volcanic  cinders,  scoriae,  glass,  132 
Volcanic  action,  effects  of,  .  .  133 
Volcanic  origin  — 

of  trappean  rocks,  .   .   .   .138 
of  granitic  rocks,    .   .   .   .138 


Volcanoes  — 

number, 26 

linear  arrangement  of,  .   .131 

near  the  sea, 129 

new,      131 

of  the  tertiary  period,  long 
active, 28 


W. 

Watt  on  fusion  of  basalt,  .   .   .100 

Waves,  action  of, 107 

Wealden,      49 

Wind  a  geological  agent,  ...    96 

Z. 

Zechstein, 47 


QUESTIONS 


TO 


ELEMENTS   OF   GEOLOGY 


14 


QUESTIONS. 


CHAPTER  I. 

SECTION  I. 

How  many  elementary  substances  are  known  ? 

In  what  combinations  is  oxygen  found  ?  What  proportion  of  the  earth's 
crust  consists  of  it  ? 

In  what  combinations  does  hydrogen  occur  ?  Nitrogen  ?  Carbon  ? 
Sulphur  ?  Chlorine  ?  Fluorine  ?  Iron  ?  Manganese  ? 

How  does  silicium  occur  ?  Aluminium  ?  Potassium  ?  Sodium  ?  Cal- 
cium ?  Magnesium  ? 

How  are  these  elementary  substances  classified  ?  (Silicium,  or  silicon, 
has  but  a  doubtful  claim  to  be  regarded  as  metallic.) 

SECTION  H. 

What  is  a  simple  mineral  ?    How  many  are  known  ? 

What  are  the  physical  properties  of  quartz?  How  are  the  several 
varieties  distinguished  ? 

What  are  the  physical  properties  of  felspar  ?  Mica  ?  Hornblende  ? 
How  are  its  varieties  distinguished  ?  Augite  ?  Hypersthene  ?  Talc  ? 
How  are  its  varieties  distinguished  ?  Serpentine  ?  Carbonate  of  lime  ? 
Gypsum  ?  Its  varieties  ? 

What  other  minerals  are  mentioned  ? 

SECTION  m. 

Define  the  crust  of  the  earth.    Rocks. 

What  are  the  unstratified  rocks  ? 

What  is  the  structure  of  granite  ? 

How  are  the  varieties  distinguished  ? 

What  is  the  porphyritic  structure  ? 

Describe  hypersthene  rock. 

What  are  volcanic  rocks  ?    Lava  ?     Scoriae  ?     Pumice-stone  ? 

How  is  the  vesicular  structure  produced  ? 

What  are  volcanic  breccias  ?    Volcanic  grits  ? 


160 


QUESTIONS. 


What  is  the  composition  of  the  trappean  rocks  ? 
What  is  the  amygdaloidal  structure  ? 

What  are  the  three  varieties  of  trappean  rocks,  and  how  are  they 
distinguished  ? 


Fig.  1. 


Name  the  stratified  rocks.  Describe  gneiss.  Mica  slate.  Sandstone. 
Conglomerate.  Greensand.  Describe  the  varieties  of  limestone. 

What  is  dolomite  ?  Of  what  does  clay  consist  ?  Clay  slate  ?  What 
modifications  does  clay  slate  present  ?  What  is  diluvium  ? 


CHAPTER   II. 

SECTION  I. 

What  is  the  primary  division  of  rocks  ? 
Upon  what  principle  are  the  unstratified  rocks  divided  ? 
Upon  what  principle  are  the  stratified  rocks  divided  ? 
Why  are  the  non-fossiliferous  called  metamorphic  rocks  ? 
Name  the  four  classes  of  rocks. 

SECTION  II. 

What  is  the  most  abundant  plutonic  reck  ? 

How  is  its  thickness  ascertained  ? 

What  is  its  amount  ? 

Where  is  it  found  ? 

What  is  its  ordinary  structure  ? 

What  peculiarity  of  structure  facilitates  the  cleavage  of  granite  ? 


QUESTIONS 


161 


14* 


162 


QUESTIONS. 


The  granitic  masses  are  generally  deep  below  the  surface  ;  in  what 
other  position  does  granite  appear  ? 


In  what  classes  of  rocks  are  granite  veins  found  ? 

Were  they  all  produced  at  the  same  time  ? 

How  is  this  demonstrated  ? 

What  is  the  relative  position  of  the  older  and  newer  granites  ? 

What  other  plutonic  rocks  occur  in  considerable  quantities  ? 


SECTION   III. 

Of  what  do  volcanic  rocks  consist  ? 

In  what  states  are  they  ejected  ? 

What  are  the  principal  varieties  of  lava,  and  how  are  they  distin- 
guished ? 

Why  is  the  basaltic  lava  the  last  to  be  ejected  ? 

How  is  the  age  of  the  volcanic  rocks  determined  ? 

What  are  the  three  divisions  of  the  volcanic  rocks,  as  dependent  upon 
age? 

What  is  the  proportion  of  the  volcanic  to  other  rocks  ? 

How  many  active  volcanoes  exist  ? 

Describe  the  eruptions  of  Kilauea. 

Describe  the  eruption  in  Iceland  in  1783. 

What  are  the  dimensions  of  Mount  ^Etna,  and  how  has  it  been  pro- 
duced ? 

How  are  the  tertiary  lavas  known  to  be  such  ? 

Where  have  they  been  most  studied  ? 


QUESTIONS. 

What  is  the  evidence  that  these  rocks  in  France  are  volcanic  ? 
Have  these  lavas  been  produced  within  the  historic  period  ? 

Fig.  5. 

X 


163 


Were  they  produced  at  an  early  period  in  the  earth's  history  ? 
Give  the  evidence  that  their  activity  was  long-continued. 

Fig.  6. 


What  is  the  form  of  the  earlier  volcanic  rocks  ? 

What  circumstances  distinguish  the  trappean  from  other  volcanic 
rocks  ? 

What  are  some  of  the  prominent  localities  of  the  trappean  rocks  ? 
How  do  they  occur  in  the  islands  west  of  Scotland  ? 
How  in  the  valley  of  the  Connecticut  river  ? 


SECTIOX  iv. 

What  is  the  lowest  metamorphic  rock  ? 

Describe  it. 

How  does  mica  slate  differ  from  gneiss  ? 

Is  it  well  distinguished  from  argillaceous  slate  ? 

What  is  the  third  rock  in  the  metamorphic  series  ? 

Why  is  it  difficult  to  determine  the  upper  limit  of  this  series  ? 


164 


QUESTIONS. 


Why  do  the  principal  rocks  of  this  series  occur  in  the  order  here 
given  ? 

What  other  rocks  may  take  the  place  of  these  principal  rocks  ? 
Where  do  the  metamorphic  rocks  occur  ? 
What  is  their  thickness  and  amount  ? 

SECTION  v. 

How  many  systems  of  fossiliferous  rocks  are  there,  and  what  are  they  ? 
What  other  system  is  provisionally  introduced  ? 
What  is  its  position  ? 

Fig.  7. 


Describe  it. 

What  materials  of  value  are  obtained  from  this  system  ? 

Fur.  8. 


What  fossils  does  it  contain  ? 


y^ 

f  UNIVERSITY   1 


QUESTIONS 


165 


In  what  localities  is  it  found  ? 

What  explanation,  in  reference  to  these  rocks,  is  given  by  those  who 
deny  that  they  constitute  a  distinct  system  ? 

Fig.  9. 


Fig.  10. 


In  what  respects  does  the  State  of  New  York  present  the  best  facilities 
for  studying  the  Silurian  system  ? 
Describe  the  Champlain  division. 

Fig.  11. 


The  Ontario  division. 
The  Helderberg  division. 


166 


QUESTIONS. 


Fig.  15. 


Fijr.  12. 


Fig.  13. 


Fig.  14. 


Fig.  16. 


QUESTIONS, 

Describe  the  Erie  division. 

What  are  the  fossils  of  this  system  ? 

Fig.  17. 


167 


Describe  the  Crinoidea. 

The  Cephalopoda,  and  the  two  forms. 

Fig.  18. 


The  Trilobite. 

What  higher  forms  of  animal  life  existed  during  the  silurian  period  ? 

The  geographical  range  of  the  system  ? 

Of  what  does  the  Old  Red  Sandstone  consist  ? 


168 


QUESTIONS, 


Describe  its  three  divisions. 
What  are  its  fossils  ? 


Fig.  19. 


Describe  the  fishes  of  that  period. 

What  was  the  peculiarity  of  the  Pterichthys  ? 

Of  the  Cephalaspis  ? 

Where  are  the  rocks  of  this  system  found  ? 

How  is  the  carboniferous  system  divided  ? 

Describe  the  carboniferous  limestone. 

What  ores  are  found  in  it  ? 

Describe,  its  fossils. 

Describe  the  millstone  grit. 

Of  what  do  the  coal  measures  consist  ? 

How  does  the  ironstone  occur  ? 

Describe  the  coal  beds. 


QUESTIONS 


169 


How  is  the  continuity  of  the  strata  interrupted  ? 
What  variations  from  this  general  type  occur  in  the  formation  ? 
Fig.  20. 

Fig.  21. 


Describe  the  several  varieties  of  coal. 
How  is  the  coal  quarried  ? 


Fig. 


What  mineral  springs  occur  in  this  formation  ? 

To  what  uses  is  coal  applied  ? 

(The  coal  was  deposited  thousands  of  years  ago,  and  has  served  no 
useful  purpose,  that  we  know  of,  till  very  recently.  Its  formation  was 
planned  and  completed  to  meet  a  want  which  was  not  to  be  felt  till  the 
lapse  of  many  ages.  It  is  a  notable  instance  of  the  wisdom  and  fore- 

15 


170 


QUESTIONS. 


thought,  as  well  as  of  the  benevolence,  of  God.)     In  what   does  this 

Pig.  25. 
Kg.  23. 


Fig.  24. 


prospective  arrangement  consist  ?     What  are  the  character  and  position 
of  the  fossils  of  the  coal  measures  ? 


Fig.  26. 


What  are  the  four  most  abundant  forms  ? 

Describe  the  Stigmaria.      The  Sigillaria.     The  Lepidodendron.     The 
Calamite. 

Where  are  the  beds  of  coal  found  ? 


QUESTIONS. 

What  is  the  fourth  formation  of  rocks  ? 

Into  what  two  portions  is  it  divided  ? 

Of  what  does  the  Permian  portion  consist  ? 

The  Trias  ? 

What  minerals  are  found  in  this  formation  ? 

What  springs  ? 

Fig.  27. 


171 


What  fossils  ? 

How  are  the  fishes  of  the  earlier  and  later  portions  distinguished  ? 

What  peculiarity  of  the  red  sandstones  is  mentioned  ? 

By  what  kinds  of  animals  were  the  tracks,  which  they  contain,  made  ? 


172 


QUESTIONS. 


Give  localities  of  the  new  red  sandstone. 
What  are  the  three  divisions  of  the  Oolitic  system  ? 
Fig.  31. 


Describe  the  Lias. 
The  Oolite. 


QUESTIONS. 

The  Wealden. 

What  are  the  general  peculiarities  of  the  system? 

Kg.  33. 


173 


What  are  the  fossil  animals  of  the  system  ? 

By  which  class  of  fossil  animals  is  the  system  characterized  ? 

•Kg.  34. 


Describe  the  Ichthyosaurus.  The  Plesiosaurus.  Pterodactyle.  The 
Iguanodon. 

Where  is  the  system  developed  ? 

What  are  the  divisions  of  the  Cretaceous  system  ? 

How  are  the  layers  of  chalk  separated  ? 

What  is  the  geological  position  of  the  Neocomian  system,  and  the 
greensand  of  this  country  ? 

What  are  the  fossils  of  this  system  ? 

Fig.  35. 


What  the  geographical  range  ? 

How  are  the  tertiary  deposits  distinguished  from  the  older  formations  ? 


174 


QUESTIONS 


Upon  what  principle  is  the  tertiary  system  divided  ? 
What  are  these  divisions  called,  and  what  does  each,  name  signify  ? 
Fig.  37. 


Fig.  39. 


Fig.  40. 


In  what  portion  of  the  tertiary  period  was  the  drift  deposited 


QUESTIONS.  175 

What  is  the  geographical  range  of  the  drift  ? 
Of  what  does  it  consist  ? 
"What  is  the  latest  tertiary  deposit  ? 
What  are  the  fossils  of  the  tertiary  system  ? 

Describe  the  Paleotherium.  The  Anoplotheritun.  The  Megatherium. 
The  Mastodon.  The  Mammoth. 

What  other  animals  belonged  to  this  period  ? 
Where  are  the  tertiary  deposits  found  ? 
What  formations  are  regarded  as  recent  ? 
What  formations  of  this  class  are  accessible  ? 
What  others  are  in  progress  ? 
What  are  the  fossils  of  this  formation  ? 

SECTION  VI. 

What  is  a  fossil  ? 

In  what  ways  are  they  preserved  ? 

When  is  a  fossil  said  to  be  mineralized  ? 

Describe  the  process  of  mineralization. 

How  is  it  proved  that  the  removal  of  the  organic  matter  and  substitu- 
tion of  mineral  particles  are  simultaneous  ? 

Were  animals  created  before  vegetables  ? 

How  is  this  shown  ? 

At  what  period  was  the  vegetable  growth  the  greatest  ? 

What  forms  of  animal  life  were  most  abundant  during  the  earlier 
periods? 

What  vertebrated  animals  belonged  to  these  periods  ? 

What  advance  is  made  in  the  new  red  sandstone  period  ? 

During  what  period  do  the  mammalia  first  appear  in  abundance  ? 

During  what  geological  period  was  man  created  ? 

How  are  the  footprints  and  skeletons  of  human  beings  hi  solid  rocks 
accounted  for  ? 

Why  are  not  fossils  distributed  uniformly  through  all  the  formations, 
and  through  all  the  parts  of  each  formation  ? 

In  what  does  the  importance  of  fossils  consist  ? 

How  are  the  fresh-water  and  marine  formations  distinguished  ? 

What  circumstances  render  it  difficult  to  identify  rocks  of  the  same  age 
in  different  localities  ? 

How  are  formations  identified  ? 

Was  the  work  of  creation  one  of  short  duration  ? 

What  was  the  last  work  of  creation  of  which  we  have  any  geological 
evidence  ? 

Why  may  we  presume  that  no  more  species  will  be  created  ? 


176 


QUESTIONS. 


Do  all  the  animal  and  vegetable  species  which  have  been  created  still 
exist  ? 

Fig.  44. 


Fig.  45. 


Fig.  46. 


Fig.  47. 


What  causes  are  operating  to  destroy  species  ? 

SECTION  VII. 

How  long  has  it  been  since  the  creation  of  the  earth  ? 

How  does  the  amount  of  stratified  rock  indicate  the  great  antiquity  of 
the  earth  ? 

How  does  the  stratification  show  the  same  thing  ? 

What  is  the  proof  that  the  principal  strata  were  deposited  before  the 
creation  of  man,  and  how  does  this  fact  bear  upon  the  question  of  the 
antiquity  of  the  earth  ? 

Give  the  argument  drawn  from  the  successive  creations  and  disappear- 
ance of  animal  and  vegetable  species. 

The  argument  drawn  from  the  amount  of  organic  matter  in  the  strati- 
fied rocks. 


QUESTIONS.  177 


The  argument  from  slow  accumulation. 

What  is  the  general  conclusion  from  these  facts  ? 

Why  is  this  conclusion  an  important  one  ? 

What  objection  to  it  has  been  raised  ? 

How  is  this  objection  answered  ? 

What  additional  explanation  is  given  ? 


CHAPTER  III. 

SECTION  I. 

What  is  the  deepest  geological  change  of  which  we  have  any  knowl- 
edge? 

What  are  the  reasons  for  supposing  that  the  lowest  stratified  rocks  are 
undergoing  fusion  ? 

Why  are  the  lowest  stratified  rocks  regarded  as  of  mechanical  origin  ? 

What  changes  have  they  undergone  ? 

SECTION  II. 

In  what  state  were  the  stratified  rocks  deposited  ? 
What  change  have  they  undergone  in  this  respect  ? 
How  is  the  fissile  structure  produced  ? 
How  is  the  cleavage  structure  produced  ? 


Fig.  48. 


What  is  the  third  class  of  changes  ? 

What  do  fractures  at  the  surface  become  ^by  the  erosion  of  water? 
How  are  caverns  formed  ? 

Describe  a  vein  of  segregation.     A  dike.     A  mineral  vein. 
What  is  a  fault? 

Were  the  inclined  strata  thus  deposited  ? 

How  is  it  proved  that  they  have  taken  the  inclined  position  since  they 
were  deposited  ? 

What  is  the  direction  of  the  dip  ? 
What  lines  form  the  angle  of  inclination  i 
What  is  the  outcrop  of  inclined  strata  ?    The  strike  ? 


178 


QUESTIONS. 
Fi<r.  49. 


QUESTIONS. 

Describe  an  anticlinal  axis.     A  synclinal  axis. 
A  valley  of  elevation.    A  valley  of  subsidence. 

Fig.  54. 


179 


When  are  strata  xinconformable  ? 

What  other  disturbances  have  taken  place  in  the  strata  ? 

When  did  these  various  disturbances  take  place  ? 

How  is  it  known  that  there  has  been  no  period  of  universal  disturbance  ? 


180 


QUESTIONS 


SECTION   III. 

How  is  it  known  that  the  mountains  have  been  covered  by  the  ocean  ? 
F«.  57. 


Fig.  58. 


QUESTIONS 


181 


Were  the  granitic  ridges  thus  covered  ? 

Has  the  level  of  the  sea  been,  to  any  .considerable  extent,  fluctuating  ? 

How,  then,  have  the  rocks,  of  which  the  mountain  masses  consist,  been 
covered  by  sea  ? 

Give  the  evidence  that  different  mountains  were  elevated  at  different 
times. 

Has  the  process  of  upheaval  been  sudden  or  gradual  ? 

How  are  the  mountain  valleys,  which  have  the  direction  of  the  mountain 
ranges,  been  produced  ? 

Fig.  59. 


How  is  the  existence  of  submarine  mountains  shown  ? 
What  is  the  movement  by  which  continents  are  elevated  ? 
State  the  evidence  of  the  elevation  of  continents  from  the  existence  of 
elevated  sea-beaches. 

The  evidence  of  the  elevation  of  the  coast  of  Maine. 
The  evidence  of  elevation  from  the  existence  of  lakes. 
16 


182 


QUESTIONS. 


From  the  geographical  range  of  the  older  strata. 

The  evidence  of  the  recent  elevation  of  South  America. 

Of  the  rising  of  the  north  of  Europe. 

State  the  proof  of  subsidence  from  the  occurrence  of  submerged  forests. 

Why  are  these  changes  but  little  observed  ? 

Fig.  60. 
1  ^~ 


What  are  the  grounds  for  asserting  that  a  change  of  level  is  taking 
place  over  a  large  area  in  the  Pacific  and  Indian  Oceans  ? 


183 


Fig.  62. 


Jig.  64. 


ifo  < 


184 


QUESTIONS. 


What  is  the  present  state  of  the  coast  of  Greenland  in  this  respect  ? 
Have  the  changes  of  level  of  the  same  place  always  been  in  the  same 
direction  ? 

Give  the  evidence  of  elevation  and  depression  in  South  America. 
In  Italy. 

Fig.  65. 


What  general  conclusion  may  we  draw  in  respect  to  the  stability  of  the 
earth's  surface  ? 

To  what  extent  can  we  ascertain  the  geography  of  past  epochs  ? 

What  former  relations  of  land  and  water  are  suggested  as  not  improba- 
ble? 

SECTION   IV. 

How  can  we  estimate  the  denudation  which  the  igneous  rocks  have 
suffered  ? 

How  do  faults  indicate  the  denudation  of  the  stratified  rocks  ? 
How  do  valleys  indicate  denudations  ? 
Describe  the  instance  in  Scotland. 


QUESTIONS. 

What  is  the  evidence  of  denudation  in  the  Connecticut  valley  ? 
How  are  valleys  produced  ? 

Fig.  66. 


185 


What  is  the  condition  of  the  surface  rock  in  the  colder  portions  of  the 

temperate  zones  ? 

Fig.  67. 


With  what  is  the  surface  rock  generally  covered  ? 

How  are  soils  formed  ? 

How  may  soils  be  improved  ? 

What  is  necessary  to  render  soils  fertile  ? 


186 


QUESTIO  NS. 


SECTION  V. 

What  means  have  we  of  judging  of  the  climate  of  former  periods  ? 

What  was  the  climate  of  the  coal  period  ? 

What  animal  fossils  indicate  a  former  warm  climate  ? 

What  evidence  that  Siberia  once  enjoyed  a  milder  climate  ? 

Do  similar  indications  appear  in  the  southern  hemisphere  ? 

When  has  the  climate  of  the  earth  been  most  uniform  ? 

Has  the  climate  been  growing  gradually  colder  to  the  present  time  ? 

What  is  the  evidence  of  a  somewhat  recent  period  of  intense  cold  ? 

What  recent  local  changes  of  climate  are  mentioned  as  having  occurred  ? 


SECTION    VI. 

State  the  general  advantages  of  geological  changes. 

By  what  changes  have  the  coal-beds  and  other  stratified  rocks  become 
accessible  ? 

What  advantage  from  these  elevating  forces  in  reference  to  the  granitic 
rocks  ? 

Fig.  69. 


How  do  these  changes  affect  our  means  of  knowing  Hhe  structure  of  the 
earth  ? 

Explain  the  origin  of  springs,  wells,  and  artesian  wells  ? 

By  what  changes  have  the  metallic  ores  become  accessible  ? 

In  what  light,  then,  are  we  to  regard  disturbances  of  geological  struc- 
ture ? 


QUESTIONS 


187 


CHAPTER  IV. 

What  is  the  object  of  the  preceding  chapters  ? 

How  can  we  arrive  at  a  knowledge  of  the  causes  which  have  produced 
geological  phenomena  ? 

Have  geological  causes  always  operated  with  the  same  intensity  ? 
How  are  the  means  of  forming  correct  geological  theories  increasing  ? 

SECTION  I. 

How  does  oxygen  become  an  agent  in  the  disintegration  of  rocks  ? 

How  does  carbonic  acid  operate  in  the  disintegration  of  rocks  ? 

What  is  the  effect  of  moisture  and  rain  ? 

What  is  the  effect  of  variations  of  temperature  ? 

What  other  atmospheric  causes  are  mentioned  ? 

How  do  these  causes  become  important  ? 

What  are  some  of  their  effects  ? 


SECTION  n. 

What  are  the  changes  which  are  to  be  referred  to  chemical  agency  ? 
Mention  some  of  the  disturbances  which  give  rise  to  chemical  changes. 
What  are  the  principal  effects  of  chemical  action  ? 
How  is  the  cleavage  structure  accounted  for  ? 

Fig.  71. 


Fig.  70. 


Mention  instances  which  show  that  a  cleavage  may  be  established  in  a 
body  in  a  solid  state. 

In  it  a  crystalline  arrangement  of  the  particles  of  the  mass  ? 
What  other  divisional  planes  exist  in  rocks  ? 
Mention  instances  of  concretionary  formations. 
Why  may  not  these  concretions  have  been  deposited  as  nodules  ? 
How  have  these  concretions  been  formed  ? 

Mention  instances  of  segregation  without  the  concretionary  structure. 
How  was  the  segregation  in  these  instances  effected  ? 


188 


QUESTIONS. 


How  is  the  columnar  structure  produced  ? 

What  is  the  origin  of  the  mineral  veins  which  are  first  mentioned  ? 

How  is  it  shown  that  other  veins  are  not  injected  ? 

How  were  these  veins  formed  ? 

What  is  the  force  by  which  these  molecular  changes  have  been  effected  ? 

SECTION  III. 

In  what  ways  are  geological  changes  produced  by  human  agency  ? 

Of  what  are  the  organic  remains,  in  rocks,  the  record  ? 

What  rocks  contain  organic  materials  in  large  quantity  ? 

What  is  the  most  abxmdant  organic  product  ? 

Explain  the  mode  of  growth  of  corals. 

Give  instances  of  extensive  coral  reefs. 

What  is  the  total  amount  of  surface  covered  by  the  coral  reefs  ? 


SECTION   IV. 

What  degree  of  importance  is  attached  to  water  as  a  geological  agent  ? 
What  are  the  sources  of  the  sediment  which  water  deposits  ? 
Why  is  not  the  formation  of  the  sedimentary  rocks  capable  of  being 
observed  ? 

What  is  the  first  mode  in  which  solid  matter  is  taken  up  by  water  ? 
Why  are  the  waters  of  the  ocean  saline  ? 

What  effect  has  the  temperature  of  water  in  the  solution  of  silex  ? 
What  effect  has  an  alkaline  condition  of  water  ? 


Fig.  72. 


What  rock  is  soluble  in  water  charged  with  carbonic  acid  ? 
Give  an  instance  of  limestone  formation  from  such  solutions. 


QUESTIONS 


189 


How  do  rivers  furnish  sediment  for  the  stratified  rocks  ? 

What  determines  the  position  of  rapids  in  rivers  ? 

"What  is  the  effect  of  water-falls  on  the  abrading  action  of  rivers  ? 

What  is  the  peculiarity  of  rock  at  Niagara  which  has  prevented  the 
fall  from  becoming  a  succession  of  rapids  ? 

What  other  circumstance  increases  the  abrading  action  of  rivers  ? 

What  is  the  principal  source  of  the  sediment  which  is  transported  by 
rivers? 

What  is  the  annual  amount  of  sediment  furnished  by  the  Kennebec  ? 
The  Merrimac  ?  The  Mississippi  ?  The  Ganges  ? 

What  is  the  general  tendency  of  these  abrading  forces  ? 

What  is  the  effect  of  waves  upon  the  coast,  when  it  consists  of  unsolidi- 
fied  materials  ? 

Describe  their  effect  upon  rocky  coasts. 

How  is  the  encroachment  upon  such  coasts  shown  ? 


What  is  the  effect  of  waves  of  less  power  ? 

How  are  marine  currents  produced  ? 

How  are  they  increased  by  the  evaporation  of  the  torrid  zone  ? 

What  are  the  most  important  marine  currents  ? 

Which  class  of  currents  have  the  greater  depth  ? 

Upon  what  does  the  power  of  deep  currents  depend  ? 

How  would  the  effect  of  these  currents  be  increased  by  earthquakes  ? 

Where  will  the  effects  of  these  currents  be  greatest  ? 

Mention  instances  of  these  effects. 


190 


QUESTIONS 


QUESTIONS 


191 


What  must  be  the  effect  of  such  currents  as  the  Gulf-stream  and 
Mozambique  channel  ? 

Mention,  generally,  the  effects  of  these  currents. 

Why  does  detrital  matter  remain  suspended  in  the  water  of  rivers  ? 

How  is  the  coarse  and  fine  sediment  separated  ? 

Why  do  river  currents  extend  some  distance  into  the  sea  ? 

What  effect  does  this  have  in  distributing  the  sediment  which  the  rivers 
furnish  ? 

Upon  what  does  the  transporting  power  of  marine  currents  depend  ? 

When  a  river  enters  a  lake,  why  is  its  sediment  deposited  ? 

Describe  the  effect. 

When  is  sediment  deposited  in  the  beds  of  rivers  ? 

Describe  the  effects  of  this  deposition. 

Where  is  most  of  the  sediment  deposited  ? 

Give  the  area  of  some  delta  deposits. 

How  do  the  deep-sea  deposits  now  forming  compare  in  extent  with  the 
earlier  formations  ? 

State  the  several  circumstances  by  which  a  succession  of  deposits  would 
be  arranged  in  strata. 

How  are  those  differences  produced  upon  which  the  separation  into 
"  independent  formations  depends  ? 

Why  are  marine  deposits  nearly  horizontal  ? 

Kg.  75. 


How  are  the  irregular  stratifications  produced  ? 

What  peculiarity  in  the  fossils  will  distinguish  the  lacustrine  and  marine 
deposits  ? 

What  peculiarity  in  reference  to  fossils  will  characterize  the  deep-sea 
deposits  ? 

How  is  coal  shown  to  be  of  vegetable  origin  ? 

Why  will  the  drift  wood  of  the  sea  accumulate  in  particular  localities, 
and  why  will  it  sink  ? 

Why  will  it  become  buried  beneath  earthy  matter  ? 

How  is  it  known  that  wood  thus  buried  will,  at  length,  become  lignite  ? 

How  is  lignite  converted  into  mineral  coal  ? 

What  is  the  proof  of  it? 

Have  beds  of  coal  been  formed  at  other  periods,  besides  the  carboniferous  ? 

Is  it  probable  that  coal-beds  are  now  forming  ? 


192 


QUESTIONS 


How  did  the  flora  of  the  carboniferous  period  differ  from  the  existing 
flora? 

Fig.  7G. 


Are  the  alternations   of   the    earthy   and  coal   strata    satisfactorily 
explained  ? 

In  what  portions  of  the  geological  series  are  the  deposits  of  salt  found  ? 
Where  is  saline  matter  principally  stored  ? 

Explain  the  conjectural  formation  of  salt  in  the  Mediterranean  Sea. 
What  form  do  rocks  take  when  deposited  from  a  chemical  solution  ? 
How  is  sand  or  gravel  solidified  by  the  infiltration  of  mineral  waters  ? 
What  is  the  effect  of  drying  upon  the  solidification  of  rocks  ? 
What  is  the  effect  of  pressure  ? 
What  of  heat  ? 

SECTION  v. 

What  is  a  glacier  ? 

What  is  the  extent  of  the  glaciers  of  the  Alps  ? 

What  change  does  the  mass  of  snow  in  the  higher  valleys  of  the  glacier 
mountains  undergo  ? 

What  is  the  source  of  supply  to  the  glacier  ? 

What  is  the  cause  of  the  motion  of  the  glacier  ? 

What  is  the  usual  annual  motion  ? 

Why  will  the  glacier  melt  but  little  at  its  under  side  ? 

Where  will  the  waste  at  the  surface  just  equal  the  addition  ? 


QUESTIONS.  193 

What  circumstances  vary  the  position  of  the  terminus  of  the  glacier  ? 
Fig.  77. 


What,  besides  snow  and  ice,  enters  into  the  composition  of  a  glacier  ? 
How  are  these  materials  supplied  ? 

How  is  a  lateral  moraine  formed  ? 

Wrhat  effect  has  the  motion  of  the  glacier  on  the  rocky  surface  over 
which  it  passes  ? 

What  is  the  material  by  which  this  effect  is  produced  ? 

How  is  the  terminal  moraine  produced  ? 

How  may  the  moraines  on  the  Jura  Mountains  be  explained  ? 

How  has  it  been  proposed  to  explain  the  striated  surfaces  of  rocks  found 
in  the  north  of  Europe  and  America  ? 
17 


194 


QUESTIONS 


What  is  the  objection  to  this  extension  of  the  glacier  theory  ? 

How  does  the  ice  accumulate  along  the  coast  in  high  latitudes  to  form 
icebergs  ? 

Why  does  it  ultimately  separate  from  the  shore  ? 

How  does  it  become  freighted  with  earthy  matter  ? 

In  what  direction  do  the  icebergs  float,  and  why  ? 

What  are  the  dimensions  of  an  iceberg,  estimated  from  the  part  that  is 
visible  ? 

Fig.  78. 


Where  does  the  mass  of  ice  increase,  and  where  diminish  ? 

What  will  be  the  effect  *of  its  melting  ? 

How  is  it  supposed  that  icebergs  may  have  striated  the  rocky  surface  ? 

What  is  probably  the  condition  of  the  bed  of  the  seas  over  which  ice- 
bergs now  float  ? 

Has  the  north  of  Europe  and  America  been  so  depressed,  during  a 
period  comparatively  recent,  as  to  admit  of  this  explanation  of  the  drift 
phenomena  ? 

SECTION   VI. 

What  is  the  condition  of  the  interior  of  the  earth  with  respect  to  heat  ? 

How  do  the  observations  made  in  deep  mines  and  wells  prove  this  ? 

How  far  is  the  temperature  influenced  from  the  surface  ? 

What  is  the  general  law  of  increment  of  temperature  ? 

At  what  depth  would  most  mineral  substances  be  melted  ? 

How  is  this  conclusion  confirmed  ? 

What  was  probably  the  original  state  of  the  mass  of  the  earth  ? 

What  other  explanation  may  be  given  of  this  interior  heat  ? 

What  is  the  elastic  force  upon  which  volcanic  phenomena  depend  ? 

Upon  what  does  the  fluidity  of  lava  depend  ? 

Upon  what  does  its  porous  structure,  when  cooled,  depend  ? 


QUESTIONS. 


195 


Why  are  volcanoes  situated  near  the  sea  ? 
Describe  the  principal  lines  of  volcanic  activity. 

What  are  the  forces  tending  to  repress  the  elasticity  of  the  mass  below  ? 
What  will  be  the  effect  when  the  elastic  is  greater  than  the  repressing 
force  ? 

What  produces  the  phenomena  of  the  earthquake  ? 

What  is  a  volcano  ? 

Why  are  volcanoes  generally  arranged  a  linear  direction  ? 

Under  what  circumstances  will  a  new  volcano  be  formed  ? 

What  instances  are  cited  ? 

How  is  a  volcanic  cone  formed  ? 

Why  are  lateral  cones  produced  ? 

How  are  volcanic  cinders  formed  ?     Scoriae  ?     Volcanic  glass  ? 


Fig.  79. 


Give  instances  of  fractures  as  results  of  volcanic  action. 
How  are  dikes  formed  ? 

Fig.  80. 


By  what  agency  have  the  changes  in  the  metamorphic  rocks  been 
effected  ? 

Give  the  instance  of  metamorphic  action  from  intrusive  granite  in 
Norway. 

What  instance  is  given  as  occurring  in  New  Hampshire  ? 


196  QUESTIONS. 

Give  the  experiment  by  which  it  is  shown  that  these  changes  will  result 
from  a  high  temperature. 

Fig.  81. 


/         '•/.*'  I         \    «H 

"/  '    \  Granite  /  ,    \     '    .     . 

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What  must  be  the  condition  of  the  lowest  stratified  rocks  in  regard  to 
temperature  ? 

Why  is  not  the  stratification  destroyed  ? 

What  changes  are  produced  by  this  high  temperature  ? 

Explain  the  connection  of  denudation  and  earthquake  action. 

What  is  the  evidence  that  the  surface  of  the  earth  is  thrown  into  undu- 
lations during  earthquakes  ? 

What  is  the  velocity  of  these  undulations  ? 

Give  the  instance  which  occurred  in  Chili. 

To  what  parts  of  the  earth  are  these  undulations  limited  ? 

What  condition  of  the  surface  may  be  regarded  as  resulting  from  this 
cause  ? 

What  is  the  class  of  rocks  most  obviously  referable  to  volcanic  agency  ? 

How  do  the  trap  rocks  differ  from  ordinary  lavas  ? 

Why  are  they  not  vesicular  ? 

Why  more  crystalline  ? 

Why  were  cones  never  formed  ? 

What  is  the  proof  that  the  granitic  rocks  have  once  been  in  a  melted 
state  ? 

Why  does  not  the  mass  of  melted  rock  below  the  surface  retain  per- 
manently its  liquid  form  ? 

Why  dees  it,  on  cooling,  become  more  crystalline  than  lava  ? 

State  the  process  by  which  mountains  are  formed. 

By  what  law  does  the  elevating  force  accumulate  ? 

Why,  then,  is  the  process  of  elevation  spasmodic,  and  not  constant  ? 

How  is  the  inclined  position  of  strata  produced  ? 

How  are  strata  brought  into  a  vertical  position  over  large  areas  ? 


QUESTIONS. 


197 


Why  do  subsidences  occasionally  follow  these  movements  of  elevation  ? 
Mention  instances. 

Fig.  82. 


»•*---'-*•  »'-' 


"What  explanation  is  suggested  of  deep  and  extensive  chasms  ? 

"What  conditions  must  exist  together,  in  the  force  by  which  continents 
are  produced  ? 

What  cause  fulfils  these  conditions  ? 

What  is  the  proof  that  the  temperature  under  given  localities  is  varia- 
ble ? 

What  will  be  the  result  of  these  variations  ? 

What  is  the  law  of  expansion  of  rocks,  as  obtained  by  experiment  ? 

What  amount  of  change  of  level  may  be  thus  accounted  for  ? 
17* 


198  QUESTIONS. 

What  circumstances  would  probably  increase  this  amount  ? 

What  amount  of  vertical  movement  must  be  accounted  for  ? 

Why  must  these  changes  of  level  be  very  slow  ? 

Under  what  conditions  would  there  be  no  change  of  level  ? 

Is  it  probable  that  these  conditions  exist  to  any  great  extent  ? 

Wrhy,  then,  are  not  the  changes  of  level  observed  ? 

Why  is  the  bed  of  the  sea  most  likely  to  experience  the  change  of  eleva- 
tion ? 

Why  are  the  continents  most  favorably  situated  to  undergo  depression  ? 

What  are  the  sources  of  heat  upon  which  climate  depends  ? 

Does  the  interior  temperature  sensibly  affect  the  present  climates  ? 

What  cause  may  be  assigned  for  the  changes  of  climate  which  are 
known  to  have  taken  place  ? 

What  are  the  relations  of  land  by  which  the  highest  temperature  would 
be  produced  ? 

How  would  this  distribution  of  land  affect  the  temperature  of  the  waters 
of  the  ocean  ? 

What  would  result  if  the  opposite  relations  of  land  and  water  existed  ? 

What  confirmation  of  these  conclusions  is  drawn  from  the  existing  cli- 
mates of  different  parts  of  the  earth  ? 

Is  there  any  reason  to  suppose  that  the  relations  of  land  and  water 
which  would  have  produced  a  warmer  climate  in  former  times  did  not 
exist  ? 


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